ON THE BRITISH NUDIBRANCHIATE MOLLUSCA. 27 



No. 2 (south),— 



Doridae 14; 



Tritoniadse 15 



— 29 



No. 3 (west),— 



Doridae 22 



Tritoniadae 20 



— 42 



The principal thing to be remarked in these catalogues is the deficiency of 

 Tritoniadce in the south and west compared with the north-eastern division. 

 That this family, particularly the genus Eolis, is a northern form, will be- 

 come still more apparent if we compare our native species with those of 

 foreign countries. The whole number of foreign species described, as far as 

 we have been able to ascertain, is 



Doridae 104 



Tritoniadae 43 



147 



of these the genus Doris contains 88 



Eolis 22 



Comparing these with the number of British species of the two families, 



DoridEB 29 Doris 18 



Tritoniadffi 46 Eolis 33 



and taking into account that a majority of the foreign species are from 

 warmer climates than our own, we see that the Doridce greatly predominate 

 in the southern and tropical seas, and the TritoniadcB, particularly the genus 

 Eolis, in the northern. Some allowance however must be made for the 

 great imperfection of our knowledge of foreign species, and the circumstance 

 that the Dorides being the largest and most conspicuous animals of the class 

 would be the first to be observed ; but that this is not sufficient to account 

 for the diff"erence will be evident if we compare the NitdibrancJiiata of the 

 Mediterranean with those of our own coast. The Mediterranean has been 

 searched by many able naturalists, and its Fauna pretty accurately ascer- 

 tained. A glance at its species of Nudibranchiata will at once show the 

 predominance of the Dorides, their superiority in number as well as in size 

 and brilliancy of colour over those of our northern climate. But if we look 

 to their Eolides, we shall, on the contrary, find them few in number and 

 small in size, and not at all to be compared with those of the British shores. 



The embryology and development of the Nudibranchiate MoUusca have 

 not until lately engaged much attention. M. Sars was the first to announce 

 (in Wiegman's Archives for 1841) that these animals undergo a true meta- 

 morphosis, and that in their young state they are inclosed in a shell, a fact 

 which your reporters have since had the opportunity of verifying in several 

 of the genera. Dr. Grant had previously published, in the Edinburgh 

 Journal of Science for 1827, an account of the development of several of the 

 MoUusca, in which he pointed out the existence of vibratile cilia in the em- 

 bryo, and their use as a means of locomotion on its exclusion from the egg ; 

 but he had failed to distinguish the peculiarities of the Nudibranchiate species, 

 as he states that there is a remarkable similarity between them and the young 

 of Buccinum and Purpura, species which do not undergo any metamorphosis. 

 The spawn of the Nudibranchiate MoUusca is deposited in the shape of a 

 gelatinous band, always arranged in a more or less spiral form, and fast- 

 ened to corallines and the under sides of stones by one of its edges. The 

 ova are minute and very numerous, amounting in some species to several 

 thousands. Before the period of exclusion, the young may be seen revolving 



