MORPHOLOGY OF THE MOLLUSCA. 265 



The sub-kingdom Molltjsca contains a vast variety of forms, representing many 

 degrees of perfection in the animal series. One section has no head, and another possesses 

 that important division of the body. The headless section comprises the Molluscoida 

 and the Acephala ; the first consisting of the classes Polyzoa, Tunicata, Brachiopoda ; 

 the second of the Lamellibranchiata. The Cephalophora, possessing a head, includes 

 the classes Gasteropoda, Pteropoda, and Cephalopoda. 



The Polyzoa are minute animals in cells, forming colonies of chitiuous or calcareous 

 polyzoaria ; each zo'id having a tentaculated mouth, a bent alimentary canal, and a nerve 

 ganglion, in a double-walled sac. The embryo, leaving the egg, is locomotive for a time, 

 and ultimately gives origin to the polypides by gemmation. 



The Tunicata are soft, marine, acephalous MoUusks, with one exception, destitute of 

 a calcareous shell, breathing by reticulate branchiae which line an internal sac, and 

 having the body enveloped in an elastic tunic, more or less dense, furnished with at 

 least two apertures. Some of the more simple forms are organically united together 

 into social assemblages ; and in one group, the Salpce, there is an alternation of 

 generations, solitary and colonial forms, succeeding each other in a cycle, as was long 

 ago described by J. C. Savigny, Chamisso, and Edwards. Many of this group are 

 likewise highly phosphorescent, the Pyrosoma, a compound form inhabiting the Atlantic 

 Ocean, being one of the most vividly luminous animals met with in the seas. The simple 

 forms, constituting the order Ascidiada, possess a higher organisation ; their body is 

 enclosed in a cartilaginous tunic lined with a muscular mantle, and having the respiratory 

 aperture surrounded by sensitive filaments^ as seen in Cynthia and Phallusia. The 

 morphology of this group has been carefully examined by Krohn, Giard, Lacaze-Duthiers, 

 Kowalevzky ; and the homology of the organs of the Tunicata, compared with those 

 of the Polyzoa, has been studied by AUman. 



The Brachiopoda are fixed bivalved animals, attached by a pedicle which passes 

 through a hole in the beak of the larger or ventral valve, and anchors it to some sub- 

 marine body ; the smaller or dorsal valve, always free and imperforate, supports on its inner 

 surface a delicate shelly loop or spiral tube, for the attachment of the peculiar organs from 

 which the name of the class is derived ; the valves are without a hinge-ligament, and are 

 joined by horny matter, as in the duck-bill shells, Lingula, or by tooth-like hinges as in 

 the lamp shells, Terebratula. The mantle is a highly vascular respiratory organ, and there 

 are usually three hearts for carrying on the circulation of the blood, the principal organ 

 lying on the stomach, and the two accessory pumps on the mantle wall. The larvae of 

 the Brachiopoda are freely locomotive, and possess eyes and ear-sacs, but these organs 

 disappear in the fixed adult state, in which the ciliated head-lobe of the embryo becomes 

 converted into the bases of the arms, which are supported on simple shelly loops, or on 

 complicated spiral tubes armed with processes or recurved spines. This class, which 

 played so important a part in the Silurian, Devonian, and Carboniferous seas, and to a 

 less extent in those of the Jurassic and Cretaceous periods, has only a few representatives 



35 



