of the Nudihranchiate Mollusca. 35 



I could not help feeling convinced that my specimen differed 

 from it in very important particulars. In Triojm, for instance, 

 there are distinct sheaths to the tentacles, which, after the most 

 careful observation, I failed to detect in my specimen. The 

 absence of branchial plumes, also, which it will be observed are 

 visible enough in the drawing of Triopa, had to be explained ; 

 while the colour of the processes, which in my drawing is bright 

 vermilion, in Triopa is orange-yellow. Still there were points 

 of resemblance yet remaining which only one so well acquainted 

 with the family as Mr. Alder would have recognized. The num- 

 ber of the processes in Triopa agreed with that of the jjapilla of 

 my specimen ; there were papular elevations on the back, occu- 

 pying the same position in both ; and the peculiar dash of 

 colour in the extremity of the foot was also common to both. 



" The tentacular sheath in the young state,^' writes Mr. Alder, 

 '•' is very small and difficult to detect." I examined the tentacles 

 long and carefully without discovering this sheath ; and I also 

 felt convinced that if branchial plumes had been present, I could 

 not have missed seeing them. But, in order to make this matter 

 certain, I wrote to my correspondent who had kindly sent me the 

 specimen, requesting to know if he had observed these plumes. 

 In reply, he said that, although he had carefully looked for them 

 immediately after having dredged it, he had failed to perceive 

 them. 



Mr. Alder's recognition of the species as Triopa claviger, 

 therefore, does not divest the specimen of all further interest, 

 inasmuch as it opens up questions of considerable importance 

 with regard to the history of the Nudibranchiata, such as the 

 following : — 



1st. How far the so-called branchiae of the Nudibranchiata 

 may be considered as breathing organs. 



2nd. To what extent colour is valuable as a specific distinction. 



8rd. The great importance of a knowledge of immature forms. 



If the terra Nudihranchiate mean anything, it means that the 

 gills or breathing organs of this order are naked, or uncovered, 

 and external. This was the character assigned to them by 

 Cuvier, in which he has been followed by the majority of zoo- 

 logists; and although they were called Opisthobranchiata by 

 Milne-Edwards, the meaning of the term is virtually the same. 

 This eminent naturalist, in 1842, was the first to remark that 

 the family Eolididse, in which the so-called branchise are papil- 

 lose, were possessed of a remarkable arrangement of the diges- 

 tive organs. He found that (as he interpreted it) the stomach 

 communicated with certain vessels, while these vessels sent off 

 branches into each of the papillse; and this gastro-vascular 

 apparatus being recognized by Quatrefages, he applied to those 



3* 



