xii PHYLUM MOLLUSCA 751 



most worm-like of Arthropods, Peripatus. Rhodnpe, moreover, 

 shows certain affinities with Flat-Worms. The head-kidneys or 

 primitive nephridia of the molluscan are practically identical 

 with those of the annelid trochosphere and are probably homo- 

 logous with the various types of nephridial tubes found in worms 

 from Platyhelminthes to Chastopoda. From developmental con- 

 siderations it appears, however, that the permanent renal organs 

 of Molluscs correspond not with the actual nephridia of Worms 

 but with their oviducts ; in other words, that they are meso- and 

 not ectonephridia. 



The fact that there is usually a single pair of nephridia and of 

 ctenidia seems to indicate derivation from a group in which meta- 

 merism had not arisen ; the segmental arrangement of the shells 

 and gills of Placophora appears to be a specialised character of 

 that group and of no phylogenetic importance. 



The lowest members of the phylum are undoubtedly the Proto- 

 branchia among Pelecypods and the Aplacophora among Amphi- 

 neura. The latter take the lowest rank in virtue of the absence of 

 both foot and shell, but the possession of an odontophore indicates 

 a comparatively high degree of specialisation. On the other hand, 

 while there is no indication of an odontophore, even in a rudi- 

 mentary condition, in the Pelecypoda, the foot and shell are well 

 developed even in Nucula and its allies. There is no actual 

 evidence to show that the foot and shell have been lost by degenera- 

 tion in the Aplacophora or the odontophore in Pelecypoda, and it 

 would appear, therefore, that the two groups are to be derived 

 independently from some primitive form. 



The facts that the pelecypod shell, at its first appearance, is 

 univalve, that the foot of the Protobranchia is of the creeping type 

 and their ctenidia plume-like, suggest the derivation of the 

 class from a form resembling a simple type of Gastropod with no 

 odontophore and with undisturbed bilateral symmetry. The Amphi- 

 neura are also bilaterally symmetrical, with paired ctenidia, kidneys, 

 and auricles, and the fact that these organs are also paired in the 

 lower Gastropoda, seems to point to a common ancestor for Pele- 

 cypods, Amphineura, and Gastropods, which was bilaterally sym- 

 metrical, had a creeping foot, a simple shell, paired auricles, 

 kidneys, and gills, and no odontophore. 



While the leading feature in the evolution of the Pelecypoda 

 has been the splitting of the mantle into two halves and the 

 resulting bivalve shell, the most noticeable fact in that of Gastro- 

 poda, apart from the appearance of the odontophore, has been the 

 torsion of the visceral mass, producing a characteristic asymmetry. 

 In the Cephalopoda, on the other hand, the primitive bilateral 

 symmetry is retained, and the most characteristic special feature 

 of the group is the extraordinary modification of the foot into arms 

 or tentacles and funnel. The class is raised far above the remain- 



