417 



POLYZOA. 



POLYZOA. 



418 



arise one from another, either singly or in pairs, from each cell (fig. 

 I5a, which represents the beginning of the polyzoary of Lepralia 

 ciliata), and from various parts of the cell, as on the back or sides, 

 near the top or not far from the bottom. And it is to the variety of 

 modes in which the cells arise that the diversity of form of the poly- 

 zoary is due. In one division of the third sub-order, the Ctenosto- 

 mata, the cells do not arise one from another, but from a tube common 

 to several cells, and which is either divided or not into distinct inter- 

 nodes. This portion of the polyzoary is termed the Basal Tube. 



Having thus defined most of the terms which it is necessary to 

 employ for the purposes of classification, we will briefly describe the 

 anatomy of the various parts of the animal in the order in which it 

 is given by Professor Allman. 



Organs for the Preservation of the Individual. 

 A. Dermal System. 



The Polyzoary is formed of a number of little chambers, or cells, 

 organically united, each of which contains a polypide, and consists of 

 two portions an internal tunic, soft, transparent, and contractile (the 

 Endocyst), and an external investment (the Ectocyst). The endocyst 

 lines the interior of the cells and when it arrives at their orifice would 

 protrude beyond the ectocyst, were it not that here it becomes inva- 

 ginated, or inverted into itself, and then terminates by becoming 

 attached round the base of the tentacular crown ; during the extor- 

 tion of the polypide it undergoes eversion, sometimes complete, 

 sometimes incomplete. The endocyst consequently constitutes a cell, 

 or sac, in which the polypide is suspended, surrounded by the peri- 

 gastric space. These sacs are all closed above where they are attached 

 to the polypide, and below, in some cases, their cavities are in com- 

 munication with those of the neighbouring naps (or with the basal 

 tube); but more generally no such communication exists. The 

 ectocyst and endocyst represent respectively the external and middle 

 tunics of the Tunicata, or the mantle and shell of other Molluscs. 



The endocyst is in all cases thin and membranaceous, and often 

 contains transverse muscular fibres. A portion, perhaps the whole, of 

 the inner surface is clothed with vibratile cilia. The ectocyst varies 

 greatly in composition and aspect. Throughout the greater number of 

 the Polyzoa it is hardened by the deposition of calcareous matter, whilst 

 in many others it is horny and flexible, and in some even of an almost 

 gelatinous consistence. In the P. hippocrepia it is in most species 

 composed of a tou<jh pergamentaceous brown membrane, strengthened 

 by the deposition of irregularly formed siliceous particles, sometimes 

 rendering it quite opaque. In other cases again, as in the genus 

 Anyuinella v. Ben., the soft and flexible, and as it were flocculent 

 ectocyst, is pervaded by aluminous and siliceous particles, and the 

 same is the case, to a less marked extent, in some other of the 

 C'tenotlomata. In Crittatdla and Pedicellina the ectocyst would, at 

 first sight, seem to be entirely absent, and the cell to be composed 

 exclusively of the endocyst. Careful examination however shows that 

 both are present, and that the ectocyst consists of a highly organised 

 and transparent tunic, free from any earthy deposit. In some 

 instances, and very distinctly in the Selenariadte, or Lunulites, the 

 surface of the calcareous ectocyst is further covered with a thin 

 horny cuticle, apparently resembling that on the shells of many 

 Molluscs. 



B. Organs of Digestion. 



These consist of an alimentary canal, commencing at the mouth 

 and terminating at the anus ; and subdivided into several portions, 

 which have received the same names as those of the apparently cor- 

 responding parts of the alimentary tract in the higher animals. The 

 mouth is edentulous and usually unarmed, though sometimes (as in 

 the proper P. hippocrepia) furnished with a valve-like organ of very 

 peculiar formation, and which is considered by Professor Allman to 

 be analogous with the 'lauguet' of the Ascidians. 



From the mouth an oesophagus, or pharynx, leads downwards to 

 the stomach, or in some cases, as in Bowwbankia, into a sort of gizzard, 

 which in that genus is armed on each side with a serrated tooth. 

 The stomach isa thick walled sac, which in most cases dilates inferiorly 

 into a rounded cul-de-sac, or pyloric cavity, from which the intestine 

 springs. The pyloric orifice is distinctly valvular, and is furnished 

 with prominent lips, which project into the intestine. The intestine, 

 wide at the origin, rapidly diminishes in diameter till it terminates 

 at a distinct anus near the mouth. The liver is represented by spherical 

 corpuscles of a brown colour, seated on the wall of the stomach. The 

 mouth and upper part of the oesophagus and the commencement of 

 the intestine", are, in most cases, at any rate, furnished with vibratile 

 cilia. 



C. Organs of Respiration and Circulation. 



Upon the tentacular crown and the walls of the perigastric space, 

 which corresponds with the ' sinus system' of the Tunicala, would 

 eem chiefly to devolve the function of bringing under the influence 

 of the aerating medium the nutritious fluid of the tissues. 



The tentacular crown of a I'olyzoon consists of two portions : 

 1, a sort of stage, or disc, which surrounds the mouth the Lopho- 

 phore ; and 2, of a row of tentacles, which are borne in an uninter- 

 rupted series round the margin of the lophophore. The lophophore 

 throughout almost the entire clan is orbicular or annular ; but iu the 



AT. HIST. DIV. VOL. iv. 



Hippocrepia its posterior margin, or that which corresponds to the 

 rectum, is prolonged into two triangular lobes, or arms, so that in that 

 order it exhibits the form of a deep crescent. This condition of the 

 lophophore is found in no marine species, and in Fredericella, a 

 fresh-water form, the arms of tha crescent are obsolete, and the lo- 

 phophore might, on a superficial view, be regarded as orbicular; but 

 a careful examination, Professor Allman goes on to say, will render 

 manifest its departure from the orbicular form, the side corresponding 

 to the arms of the crescent being slightly prolonged obliquely upwards. 

 In all cases the lophophore forms the roof of the perigastric space ; 

 in the species with crescentic lophgphores the interior of the arms is 

 clothed with vibratile cilia. 



The tentacles are tubular, closed at their free extremity and opening 

 at the opposite end through the lophophore into the perigastric space. 

 In all the Polyzoa, they are armed upon their opposed sides (ia Pedi- 

 cettina on one only ?) with vibratile cilia, arranged in a single series. 

 and vibrating towards the extremity of the tentacle upon one side 

 and towards the base on the other. A nervous filament and muscular 

 fibres may be traced iuto the tentacles. In the proper P. hippocrepia 

 the entire plume of tentacles is surrounded at its base by an exceed- 

 ingly delicate transparent membrane in the form of a cup or calyx, 

 considered by Professor Allman as analogous to the membrane of the 

 respiratory sac in the Tunicata; but this calyx has not yet been 

 detected in any marine polyzoon. In the genus Pedicellina the ten- 

 tacles are also surrounded at their base by a kind of membranous calyx, 

 but this is of an entirely different import from the membrane con- 

 necting the bases of the tentacles in the P. hippocrepia. 



The perigastric space and the interior of the lophophore and ten- 

 tacles all freely communicate with one another, and are filled with a 

 clear fluid, in which float numerous particles of a very irregular form 

 and size. This fluid obviously represents the blood or common nutrient 

 and respiratory fluid of other Molluscs. It is kept in motion by the 

 cilia with which the endocyst is lined, but there is no special circu- 

 latory organ as iu the Atcidite. 



D. Organs of Motion. 



The muscular system in the Polyzoa is highly developed, and the 

 muscles are especially interesting in a physiological point of view, for 

 they seem to present an example of true muscular tissue reduced to 

 its simplest and essential form. They are composed of bundles of 

 elementary fibres, totally separate from each other, throughout their 

 entire course, and which are distinctly marked with transverse strisc. 

 They resemble in fact very closely the fibres of the thoracic muscles 

 of insects. In the marine Polyzoa however another kind of fibres may 

 be noticed, presenting nodular enlargements, which would seem to 

 resemble very closely the isolated, organic muscular fibres of the 

 higher animals. The muscles are disposed in distinct sets, and it is 

 by the agency of these various groups that the different movements of 

 protrusion and retraction of the polypide are effected, together with 

 the actions of the tentacles and of the avicularia and vibracula. For 

 the arrangement of the muscles in the former class of organs see 

 jig. 2 (5) iu the article MOLLUSCA. The curious analogies in the dis- 

 position of these muscles in the Polyzoa with that of the muscles 

 which act upon the shells of the Brachiopoda are also pointed out iu 

 that place. 



E. Organs of Life of Relation. 



A distinct nervous system was first shown to exist in the Polyzoa by 

 M. Dumortier in Lophojiun cryttallinus, and has been demonstrated 

 by Van Beneden in Laguncula, and by Allman in all the Hippocrepian 

 genera except Paludicella ; it may be deemed therefore to exist 

 generally in the class, and will probably be found essentially alike in 

 all. In all the species of the Hippocrepian order there may be seen, 

 attached to the external surface of the oesophagus, on its rectal aspect, 

 just below the mouth, a hollow oval body of a yellowish colour, which 

 is undoubtedly a nervous ganglion, as Professor Allman has succeeded 

 iu distinctly observing nervous filaments in connection with it ; some 

 of which may be traced going to each tentacle. The ganglion also sends 

 off filaments upwards towards the mouth, and one may be observed 

 passing downwards along the oesophagus ; but nothing like a complete 

 collar surrounding the tube has been observed. The Polyzoa do not 

 seem to possess any special organs of sense. 



F. Organs of Locomotion. 



In CrittalMa, the ectocyst, according to Professor Allmau, is highly 

 contractile, and presents, below, a flattened disc destitute of apertures. 

 Upon this disc, which closely resembles the foot of a Gasteropod, the 

 singular polyzoary creeps about upon the stems and leaves of aquatic 

 plants. Except in the embryonic condition no other Polyzoon would 

 seem to possess any power of locomotion ; or at any rate none has 

 been noticed, but several reasons would seem to render it probable 

 that the species belonging to the Selenariadie may be capable of loco- 

 motion by means of their curiously-constructed vibracula. 



0. Reproduction. 



In the Polyzoa, observes Professor Allman, three distinct modes 

 of reproduction may bo witnessed, namely: By buds or geiurnte; 

 by true ova ; and by free locomotive embryoes. 



1. Reproduction by Oemmso. The gemmae always originate in 

 the eudocyat, firsfc appearing as smull tubercles projecting iuto the 



2 E 



