28 REPORT— 1844. 



on their own axis by means of vibratile cilia, and on escaping from the egg, 

 they swim about freely in the water by the same means. The larva is ex- 

 tremely minute, and has more the appearance of a rotiferous animalcule than 

 a MoUusk. It is inclosed in a transparent, calcareous, uautiloid shell, with 

 an operculum. Its structure is very simple, showing no signs of the external 

 organs that distinguish the future adult. The principal portion visible out- 

 side the shell is composed of two flat discs or lobes, fringed with long cilia, 

 by the motion of which it swims freely through the water. These are often 

 withdrawn into the shell, and the operculum is closed upon them when the 

 animal is at rest. We have not yet been able to trace the animal further 

 than the first stage of its development, and are therefore unable to say by 

 what process it assumes the very different form of the adult state. We have 

 succeeded in bringing out the larvae of Doris, Tritonia, Melibcea, and Eolis, 

 between all of which there is a very great resemblance. The embryology of 

 the Mollusca has been so little investigated, that it would be difficult to point 

 out the alliances that this mode of development appears to indicate. M. Van 

 Beneden has shown the existence of a similar larva in Aplysia, and it is pro- 

 bable that Bulla and some others of the Tectibranchiata will be found to 

 follow the same type. The majority of those Gasteropoda whose embryology 

 is known do not undergo any metamorphosis. The ciliated discs observed 

 in the young of Buccinum and Purpura after birth cannot be considered an 

 exception, as they disappear almost immediately, and the shell and other 

 organs with which the young animal is furnished on its exclusion from the 

 egg are essentially the same that it retains to the latest period of its existence. 



The anatomy of the Doridce was carefully studied by Cuvier, and found 

 by that distinguished naturalist to agree in all important characters with the 

 true molluscan type. 



The Eolidians, however, which comprise most of the Tritoniadce, vary in 

 this respect from the rest of the order. M. Milne-Edwards was the first to 

 draw the attention of physiologists to the fact, and more recently M. de Qua- 

 trefages has investigated the subject with great elaboration. 



In most of the Gasteropodous Mollusca the liver is largely developed, but 

 in this division of the Nudibranchiata that organ entirely disappears from 

 the abdomen. At the same time a system of vessels is found to exist in 

 connection with the stomach, and branching into the dorsal papillae, the in- 

 terior of which is clothed with a coloured glandular substance, which probably 

 acts the part of a liver and contributes to the digestive process. This system 

 of vessels has been called gastro-vascular, and is stated to receive the more 

 refined products of digestion immediately from the stomach. It is compared 

 by M. Milne-Edwards and M. de Quatrefages to the circulatory system of 

 the Medusidm on the one hand, and of Nymphon and some of the Annulosa 

 on the other. It appears, however, according to our observations, to be merely 

 an appendage of the digestive system, while the vessels of the Medusida unite 

 the two functions of digestion and circulation into one. The circulation of 

 the blood is provided for in the Eolidians by a separate system of vessels, con- 

 sisting of a heart and arteries ; but according to M. de Quatrefages the veins 

 disappear in his genus Eolidina, their place being occupied by lacunae. The 

 respiratory function resides chiefly in what are called the branchial papillae. 

 The skin, however, considerably assists in aerating the blood. This function 

 is therefore more diff"used than is usual in the Gasteropoda, in most of which 

 respiration is provided for by highly developed branchiae. In the typical 

 Doridce the branchial plumes are of a very elaborate character, but we may 

 perceive in some of the thin-skinned genera of that family, as in Pob/cera, 

 an indication, by the presence of vibratile cilia over other parts of the body, 



