40 THE VOYAGE OF H.M.S. CHALLENGER. 



them as great separate groups of distinct origin. Neumayr seems almost alone in this 

 opinion. On the contrary, I regard the Desmodonta as derived from certain Heterodonta, 

 and the close resemblance in the structure of the two series prevents me from separating 

 them. 



Finally, it will be seen that the forms with rudimentary gills belong to the 

 terminal groups : Clavagellidae, Cuspidariidse, Poromyidse, Tellinidse, Lucinidse, which is 

 contrary to the opinion of Von Jhering, who regarded the latter as primitive.' But 

 these terminal groups have distinct origins, as the phylogenetic table suggests, and 

 cannot possibly be united in any natural classification. 



111.— CONCLUSIONS. 



As the subject is still new, and hitherto but little investigated, we cannot yet 

 presume to formulate general and final conclusions. But few deep-sea Molluscs have 

 as yet been studied, and many researches must still be undertaken before definite 

 generalisations can be ventured. 



Having made these reservations, I think, nevertheless, that in order to sum up the 

 facts discussed above it is necessary to attempt to formulate certain results. From the 

 observations which I have made on the MoUusca ^ from the deep sea, I conclude : — 



1. An organ of special sense, the organ of vision, may atrophy and disappear, in 

 consequence of the absence of sufficient light in the great depths : Guivillea and 

 certain forms of FJeurotoma, Fossaryf:{l), and Pvncturella,. among Gastropoda; 

 Amusium among Pelecypoda. 



2. Correlatively, the organs of general sense may multiply and acquire a high 

 degree of development : labial palps of Trochus infundibulum ; siphonal tentacles of 

 varied structure in the deep-sea Anatinacea and in Malletia. 



3. The respiratory activity may diminish and the gills become rudimentary in 

 various ways (certain Anatinacea : Poromyidse, Cuspidariidse ; Cryjptodon), or may retain 

 great simplicity of structure {Malletia). 



' Vergleichende Anatomic des Xervcnsystemes und Phylogenie der MoUusken, p. 64. 



2 These results, especially the first and second, agree with what has been observed of other animals from the deep 

 sea, such as Crustaceans and Fishes. 



