290 



MUSEUM OF ANIMATED NATURE. 



[TUNICATA. 



CLASS TUNICATA 



(Lu Acephalet sons coquiltes of Ciivier ; Acephalo- 

 phora heterobranchiata, De Blainville). The Tuni- 

 cata are raollusks enclosed not in shells, but in a 

 sac or investment of a cartilaginous nature, and more 

 Or less flexible. 



In the opinion of many naturalists the Tunicata 

 form an intermediate link between the Conchifera, 

 or bi-valve mollusks,onthe one hand, and the Acrifa 

 (Polypes, jelly-fish, &c.) on the other. Their re- 

 lation, says Mr. MacLeay, to the testaceous mol- 

 luaca has been pointed out by Aristotle, Basta, Lin- 

 lueus, Pallas, Cuvier, and Savigny : their relation to 

 the polypes has likewise been shown by Savigny, 

 when he demonstrated that the Alcyonium ficus of 

 Unnsus is nothing else than an aggregation of 

 minute Ascidise combined in a common envelope. 



Strange and varied are the fcrms under which the 

 Tunicata meet the eye of the naturalist, and diverse 

 are their modes of life. Some, as the Ascidiae, are 

 fixed to the rocks ; others, as the Salpae, are free 

 and swim about ; others, as the Pyrosoma, are aggre- 

 gated together, and form a sort of compound whole, 

 floating with the current of the ocean. Others, 

 again, clustered together and appearing like the 

 rays of a star, as the Botrylli, are adherent to the 

 bodies of other mollusks, or Ascidio! and sea-weeds. 

 See Fig. 3031, a group of Botryllus stellatus upon 

 Ascidia intestinalis. 



Cuvier divides the Tunicata into two sections : 

 the first comprehends such as are isolated, that is, 

 not organically aggregated to others of the same 

 species, although they are often found clustered 

 together ; the second contains aggregated species, 

 viz. those which are united into a common mass, 

 organically bound together, as we shall find in many 

 of the Acrita, or zoophytes. 



In order that we may understand the general 

 structure of the isolated Tunicata, let us select the 

 Ascidia as an example. 



The Ascidiae (divided into several genera and 

 subgenera) are fixed to rocks and stones ; and, ex- 

 cepting by expansions and contractions, and by the 

 forcible ejection of the water, they show but few ex- 

 ternal signs of life. In their appearance there is 

 nothing very attractive ; externally viewed they 

 seem mere fleshy masses, with little to recommend 

 them to our notice ; but beneath this exterior will 

 the zoologist discover a strange and beautiful or- 

 ganization, claiming the most minute scrutiny. 



Let us suppose one of the fleshy-looking creatures 

 of an oblong or sac-like form (see Fig. 3031, repre- 

 senting two specimens of Ascidia, one anterior to 

 the other), fixed by its base to the surface of the 

 rock. On examining it with a little attention, we 

 shall observe an orifice at its very summit, and 

 below this, again, a branching or prominent part, 

 with an orifice also at its termination. Of these 

 orifices the uppermost is the injestive orifice, leading 

 to a delicate reticulated sac, performing the office 

 of branchiae. The lower or lateral orifice is the 

 ejestive aperture. 



Now the tunic or outer investment consists of a 

 muscular layer with an epidermic membrane, and a 

 delicate vascular serous lining, which at the orifices 

 is reflected over the body of the enclosed animal ; 

 thus it hangs loosely, as it were suspended, in its 

 sac-like case, by means of the reflected part of this 

 peritoneal tissue. 



We have said that the upper orifice leads to a 

 delicate respiratoiy sac beneath the outer case or 

 tunic ; this sac is of circumscribed extent, very vas- 

 cular, being covered with a network of vessels of 

 infinite minuteness, and has its inner surface pro- 

 vided with minute and countless vibratile cilia, ever 

 in action, and keeping up a perpetual current in 

 the water imbibed. 



The external orifice is surrounded by tentacula 

 of great sensitiveness, which ascertain the presence 

 of particles unfit for admission ; the aperture thus 

 rejecting some, but allowing the ingress of others; 

 for, strange to say, the entrance to the oesophagus, 

 that is, the real mouth of the animal, is at the bottom 

 of this branchial sac. It appears that the nutritive 

 particles taken in with the water are deposited on 

 the ciliated surface, which they are made to traverse 

 till they merge into a descending stream, which 

 flows to the cesophagus, and thence into the sto- 

 mach. 



The stomach is simple, and receives through 

 several orifices the biliary secretion : the liver is a 

 glandular mass adherent to the stomach and ali- 

 mentary canal. The latter is folded upon itself, 

 and terminates, alter emerging from the peritoneal 

 investment, in a cavity communicating with the 

 lateral orifice. 



The circulating system is simple ; the nervous 

 system at a low degree of development. The egg- 

 sac terminates in the same sac as the alimentary 

 canal. The Ascidiae are found in all seas, and 

 valued in China as articles of food. 



As an example of the Ascidian family we have 

 already referred to the Ascidia intestinalis (Fig. 

 3031), and we may enumerate the genera Phallusia, 

 Cynthia, Clavellina, BipapiUaria, &c. To this 

 family must be referred the Boltenia reniformis 

 (Fig. 3032), which presents a somewhat oval body 

 or sac, aflixed to the rock by means of a long slender 

 peduncle. It is found in the northern seas of Ame- 

 rica. Referring to the figure, A is placed at the 

 ejestive orifice ; C, at the branchial orifice ; P, at 

 the peduncle. 



Another genus is Dendrodoa, MacLeay, of which 

 one species, Dendrodoa glandaria, is given at Fig. 

 3033 : a represents the animal of the natural size : 

 the base is incrusted with pebbles, and resembles 

 the cup of an acorn ; b, the same seen obliquely, so 

 as to show the top, which is a little compressed. 

 The orifices are minute. 



In this family the Fodia rubescens (Fig. 3034) is 

 also placed. It is of an oval shape, and is attached 

 to stones by means of a' kind of sucker. It is found 

 on the coasts of North America. 



We may also here notice the Cystingia Griffithsii, 

 MacLeay (Fig. 3035), which in many particulars 

 approaches the Boltenia. The peduncle is evidently 

 the portion by which the animal is fixed. It is 

 found in the northern seas of America (Winter 

 Island, ' Captain Parry's Third Voyage '). A, the 

 animal of tne natural size, see on the right side ; 

 B, the same magnified, seen on the left side ; a, ejes- 

 tive orifice ; b, branchial orifice ; s, grains of sand 

 externally incrusting the thick end of the pe- 

 duncle. 



It is, perhaps, to this family that we must refer 

 that singular animal the Chelyosoma MacLeayanum, 

 Brod., a native of the Arctic seas, where it is found 

 adhering to stones. The coriaceous envelope is lami- 

 nated above, and modified into divisions like the 

 back shell of a tortoise ; and consists of eight some- 

 what horny angular plates, of which three surround 

 the branchial orifice, and four the other. This spe- 

 cies is represented at Fig. 3036 : a, side view ; 

 b, seen from above ; c, the interior of the upper 

 plate. 



Another family of the Tunicata is that of the Sal- 

 paceans (Les Biphores, Brug. ; Thalia, Brown ; 

 Salpa and Dagysa, Gmel.). This family compre- 

 hends numerous species, many of which are so 

 transparent that it is not always easy to discern 

 them in a body of sea-water. They are free, not 

 fixed, and are enclosed in a gelatinous cartilaginous 

 tunic, of an oval or cylindrical form, and open at 

 both extremities : that at the posterior extremity is 

 large and furnished with a valve, which permits the 

 entrance of the water, but prevents its exit, which 

 takes place only at the anterior orifice. Hence by 

 a series of dilatations and contractions, the water 

 entering at one orifice, and being forcibly expelled 

 at the other, the animals propel themselves along, 

 the hind part foremost. Within the tubular cavity 

 or canal through which the water passes is a vas- 

 cular riband, attached by both its extremities to 

 the walls of the canal, so that it is perpetually laved 

 by the current as it passes ; this vascular riband is 

 the branchial apparatus. The viscera occupy a 

 small cavity between the canal and the dorsal 

 portion of the tunic near the mouth, which opens 

 adjacent to the upper extremity of the branchial 

 riband. In many species the tunic exhibits the 

 most brilliant colours of the rainbow, while its trans- 

 parence permits the internal parts to be perfectly 

 visible. 



The most extraordinary fact in the history of these 

 yet imperfectly understood mollusks is that stated 

 by Charaisso. According to this observer, Salpae 

 are found swimming in chains, the individuals com- 

 prising the chain adhering to each other apparently 

 by means of minute suckers, but probably without 

 organic union. These adherent Salpae give birth to 

 individuals of very different form, which are always 

 isolated, but which produce concatenated Sal pie, 

 and these concatenated Salpse again produce iso- 

 lated beings, and so on in alternate succession, ge- 

 neration after generation. Cuvier, who quotes Cha- 

 migso, says, " It is certain that in some species may 

 be seen young individuals in the interior of their 

 parents, adhering together by means of a sort of 

 minute sucker, and differing in form from the ani- 

 mals containing them." We have still much to 

 learn of the economy of organic beings 1 The Sal- 

 paceans are found in the Mediterranean and the 

 hotter latitudes of the ocean. Many are beautifully 

 phosphorescent. Among the genera of this family 

 may be mentioned Holothuria, Monophorus, Phyl- 

 liroe, &c. 



Referring to our pictorial specimens. Fig. 3037 

 represents the Salpa firoloVdea (genus Timoriensis, 

 Quoy and Gaim.). It does not appear to be con- 

 catenated : the anterior extremity is very elongated 

 and pointed. It is found in the seas of Timor, &c. 

 Fig 3038 represents the Salpa polymorpha, Quoy 

 and Gaim. It is angular and recurved, with the 

 terminal orifices very much approximated. Whether 



it becomes concatenated or not is yet to be ascer- 

 tained. The species is figured in two views. 



Fig. 3039 represents one of the concatenated spe- 

 cies, Salpa fusiformis, of which several are shown 

 united together. The concatenated Salpis form 

 chains, often of many yards in length, ana, as they 

 are subject to the undulation of the waves, float 

 along with a serpentine motion ; the contractions 

 of each individual are synchronous, and thus the 

 chain winds its way over the surface of the sunlit 

 ocean, like a long-drawn riband of silver. 



Fig. 3040 represents the Salpa Zonaria, also one 

 of the agglutinated species. The upper specimen 

 shows several individuals united together, and below 

 are two individuals disunited. 



We may now pass to the Aggregated Tunicata, 

 Les Aggreges of Cuvier. 



The Aggregated Tunicata are not united to each 

 other by mere agglutination, but are conglomerated 

 together into a single organic mass — a compound 

 unity, and in this respect remind us of the coral 

 polypes. Nevertheless it would appear from the 

 observations of MM. Audouin and Milne Edwards, 

 that the individuals of which any aggregated mass 

 consists were produced from the egg distinct, and 

 that they lived thus and swam about, not unlike 

 tadpoles in form, vibrating a slender tail. After- 

 wards, however, at a certain period of their exist- 

 ence, they undergo a sort of metamorphosis, and 

 numbers uniting together constitute one aggregated 

 whole. In structure they approach more or less to 

 the Ascidian mollusks. Some, however, as the 

 Botrylli, with an opening at each extremity, may be 

 regarded as aggregated Salpae. 



In the genus Botryllus the separated animals are 

 of an oval form, and ten or twelve united together 

 form a sort of star, which is fixed upon other bodies, 

 and often on the Ascidiae. The branchial orifices 

 are at the extremity of the rays of the star, and the 

 other orifices open into a common cavity in its 

 centre. If the orifice of one individual be irritated, 

 that animal exclusively contracts ; but if the centre 

 be touched, all contract together. Referring to 

 Fig. 3031, several stars of the Botryllus stellatus are 

 seen on a specimen of Ascidia intestinalis, a; 2> re- 

 presents one of the stars or discs magnified. 



Another group is represented by Pyrosoma, cele- 

 brated for its phosphorescence. 



These singular creatures present the appearance of 

 a simple gelatinous tube, rather larger at one end 

 than the other. The tube varies in different species 

 from one to six inches in length ; it is hollow, with 

 a distinct aperture at the larger end, and, as Cuvier 

 and Rang say, closed at the other. Mr. George 

 Bennett, however, describes the tube as open at 

 both ends, the orifice at the smaller extremity being 

 more contracted than that at the other. 



Now the substance of this tube, if examined, will 

 be found to consist of a multitude of buds, or gem- 

 mules, closely set together in a common gelatinous 

 tissue ; these buds are all distinct animals, each 

 with its branchial orifice and viscera, and thus or- 

 ganically united, the tubular aggregation swims on 

 the sea, by the combined contractions and dilations 

 of the multitude working in unison, a continuous 

 stream of water being propelled through the tube. 

 Several species are deswibed, as P. Atlanticum, P. 

 elegans, P. giganteum, &c. Enveloped in a flame 

 of bright phosphorescent light, and gleaming with a 

 greenish lustre, the Pyrosoma presents a most bril- 

 liant appearance, and when seen at night in vast 

 shoals upwards of a mile in breadth, and stretching 

 out till lost in the distance, the glory of the spec- 

 tacle may be easily imagined. The vessel, as it 

 cleaves the gleaming mass, throws up strong flashes 

 of light, as if ploughing through a liquid fire, which 

 illuminates the hull, the sails, and the ropes with a 

 strange unearthly radiance. 



After death the splendour of the Pyrosoma van- 

 ishes, and gives place to a dull pale yellowish white. 

 Fig. 3041 represents the Pyrosoma giganteum; a, a 

 portion magnified. 



Another group or family of this section, compre- 

 hending several genera, as Polyclinum, Synoicum, 

 &c., presents us with species forming by aggregation 

 fleshy masses, incrusting or rooted upon other 

 bodies : some are elevated, the distinct animals 

 forming branches ; others have a sponge or fungus- 

 like figure. 



Fig. 3042 represents the Synoicum ficus: a, a 

 portion highly magnified, showing the several indi-' 

 viduals. Fig. 3043 represents the Synoicum tur-' 

 gens. Both are natives of the European seas. 



Fig. 3044 represents an example of the genus 

 Amaroucium, namely, A. proliferum, of the natural 

 size. It is found along the coasts of the British 

 Channel. Fig. 3045 represents a portion of the 

 Amaroucium proliferum magnified. 



We have already said that the young of the 

 Aggregated Tunicata are free and independent, and 

 not unlike the tadpole of the frog in form, though 

 of extreme minuteness. Fig. 3046 represents the 

 young of the Amaroucium proliferum highly mag- 



