1186 



TUNICATA. 



teries to veins and its veins to arteries. The 

 Ascitlians, too, furnish the first examples of 

 complete metamorphosis in the lower ranks 

 of the animal kingdom. The honour of this 

 discovery is due to MM. Audouin and 

 Milne-Edwards. From the late discoveries 

 of M. Sars, these metamorphoses increase in 

 interest, and appear to he still more remarkable. 

 And, lastly, the Ascidians have contributed 

 very considerably to our knowledge of the cir- 

 culatory apparatus of the Mollusca generally." 



The term Tunicata was first used by La- 

 marck ; its synonyms are Tethya, Auct. ; Soft- 

 shelled Molluscs, Hunter ; les Aceplialcs sans 

 coquilles, Cuvier ; Acephalophora heterobran- 

 chiata, Blainville ; Tunicaries, Kirby ; Gymna- 

 cephala, Bronn. The CLASS of animals to 

 which it is applied may be zoologically de- 

 fined as consisting of acephalous Molluscs ; 

 with a soft shell or test, organised, coriaceous 

 or gelatinous, frequently destitute of mineral 

 constituents, having a large proportion of 

 cellulose in its composition : animals single 

 or aggregate; the test of each animal 

 provided with two apertures, one bran- 

 chial, the other anal ; the mantle forming 

 an interior coat ; the branchiae attached 

 wholly or in part to the internal surface of 

 the mantle ; the mouth, without labial ten- 

 tacles, placed below the branchial apparatus ; 

 animals hermaphrodite, undergoing metamor- 

 phosis in their young state. 



The cavity, whether of single or compound 

 Tunicates, is occupied by a more or less 

 muscular sac, provided, like the external tunic, 

 with two orifices. This sac, identical with 

 the " mantle " of the Acephalans, is attached to 

 the inner surface of the test, generally only at 

 the orifices, and contains the viscera. The di- 

 gestive, reproductive, and circulatory organs are 

 disposed at the base of the sac, and its upper 

 and larger portion, lined with, or traversed by, 

 the branchiae, forms the branchial cavity. This 

 is placed at the commencement of the alimen- 

 tary canal, of which it forms as it were the 

 antechamber. The branchia? have generally 

 the form of ridges, more or less complicated, 

 and seldom symmetrical. The alimentary 

 canal is simple, and barely distinguishable into 

 gullet, stomach, and intestine. It is always 

 convoluted or folded once on itself. The 

 liver adheres to the stomach, and in many 

 species is divided into distinct lobes. The 



heart consists of a slightly bent, contractile 

 tube, and is situated near to or within the in- 

 testinal loop. The reproductive organs, con- 

 sisting of ovary and testicle, are often lodged 

 in the fold of the intestine. 



The animal of these "soft-shelled" Mol- 

 luscs has very close affinities with that of the 

 other Acephalans, especially the lamellibran- 

 chiates. And " were the test of an Ascidian 

 converted into a hard shell, symmetrically 

 divided into two plates, connected together 

 dorsally by cartilage, and capable of separation 

 so as to expose the mantle along a ventral 

 mesial line, whilst the orifices protruded at 

 one extremity, it would present the closest 

 similarity with many bivalve Molluscs."* 

 (Forbes.) 



All the Tunicata are free during the earlier 

 periods of their existence ; some remain per- 

 manently free, floating in the water (Salpce, 

 &c.) t but the generality (BotrylIi,Ai>cidi(e,&c.) 

 become fixed to shells, seaweeds, and other 

 marine bodies. Some exist as distinct indivi- 

 duals (Ascidia, Cynthia, &c.), whilst various 

 degrees of combination are affected by others 

 (Botryllus, Clavellina, Pyrosoma, &c.) ; and 

 some are simple in one generation and com- 

 bined in the next (Salpa). 



From the above-mentioned various con- 

 ditions of individualism or aggregation under 

 which these animals exist, the family is 

 divisible into two groups the simple and 

 the aggregate. Such forms of the latter 

 group as were known to the earlier natu- 

 ralists were, from a general similarity of 

 appearance, classed by them with Alcyoma. 

 In the former group have been placed, until 

 a late date, the numerous species of the Sal- 

 p'ldfB, which now however, like the above- 

 mentioned groups, have been separately treated 

 of and illustrated in memoirs and monographs 

 devoted to them. Indeed, although the ex- 

 pressions, simple and aggregate, as will be seen 

 hereafter, are very convenient in describing 

 the physiology of the Tunicata, yet late re- 

 searches have shown that the conditions to 

 which they refer have but a subordinate value 

 in reference to the natural grouping of the 

 class. 



As may naturally be inferred from what is 

 stated above, the* Tunicata have been sub- 

 divided in various manners. The arrangement 

 we are inclined to adopt is as follows : 



TUNICATA 



Dichitonida, Fleming. 



Inner sac more or less detached 

 from the external tunic ; 

 united to it at the two ori- 

 fices. Branchiae, large, equal, 

 spread on the inner surface of 

 this sac. Branchial orifice 

 with an inner membranaceous 

 ring and circle of tentacles. 



Monochitonida, Fleming. 



Inner sac adhering throughout 

 to the external tunic. Orifices 

 without tentacular fringes. 



Fixed. Branchial and anal 

 orifices not 



Floating. Branchial and! 



anal orifices opposite to I -r> i 



each other. Branchiae f P ^ rOsomidae ' 

 disjoined. 



Branchial and anal orifices 1 c , . , 

 opposite to each other. J Sal P ldlE - 



Branchial and anal orifices'] 



not opposite to each >- Pelonaiadae. 

 other. 



* We may particularly mention the Mycs, PhoJaJes, &c. 



