THE NAUTILUS. 81 



It is seen that the PTEKOPODA have disappeared as a division of 

 higher order; they are ranged under two tribes of the sub-order 

 Tecfibranc/tia, order Opisthobranchia, of Gastropoda (pp. 170 and 

 173). 



Many conchologists and zoologists may be surprised to find the 

 class Lamellibranchia * ranged under the same group with the Gas- 

 tropoda and Scaphopoda, as in contrast to the Isopleura and the 

 Siphonopoda. It still appears that the arrangement as adopted by 

 Lankester and other zoologists, is more natural : Lipocephala or 

 Acephala, and Cephalophora. Not alone is the presence or absence 

 of a head a distinguishing feature. On pp. 67 the author says : 

 " the radula is characteristic of the phylum mollusca. It exists 

 throughout the series . . . and is only absent in the most specialized 

 types, in which it has evidently been lost, such as ... the Larnelli- 

 branchs . . ." If it was lost in the latter there should be traces of 

 it in the embryo, the more so since the embryonal and larval stages 

 of at least many Lamellibranchia are of very well marked forms and 

 existing as such through a long time and under various conditions. 

 Also for other reasons it might appear that the Lamellibranchia are 

 not a specialized or retrograde group, e, g., from the Gastropoda or 

 some primitive form near them, as must be inferred from Pel- 

 seneer's arrangement, but one primitively different. At any rate, I 

 believe that they represent a group of decidedly inferior organiza- 

 tion, and that their proper place is not in the same group with the 

 Gastropoda, and between the latter and the Cephalopoda. The 

 formation, and especially the functions of the ctenidia (branchiae), 

 are certainly significant ; 2 and although the Lamellibranchiata have 

 been decidedly, and as it seems, definitely separated from the 

 Bryozoa, etc., the fact should not be overlooked that the branchias of 

 the former have not only the same functions principally nutritive 

 as the tentacles of the latter, but that, in many instances at least, 

 the filaments are of similar formation, even to minute details. 

 Something else might be said in this connection : it is the tendency 

 of our day to found classification on a single organ, or organ sys- 

 tem rather than on the ensemble of the whole organization. 



1 On p. 197 the author says that the name Scaphopoda has been more gen- 

 erally used than Solenoconc/ta, for the sake of uniformity ; for that same reason 

 he might have adopted the name Pelecypoda in place of Lamellibranchia (better: 

 Lamellibranchiata) . 



2 Confer also Lankester 1. c., pp. 684 and 685. 



