MICROSCOPICAL STUDIES. 23 



being supplied from the brain (cerebral ganglia) in the Pteropods, 

 and from the pedal or foot ganglia in the Cuttles. Hence it seems 

 much more likely that the resemblances between the Pteropods and 

 Siphonopods are rather homoplastic than homologous, i.e., have arisen 

 independently rather than being possessed of common origin. Like 

 circumstances not infrequently produce analogous shapes and organs 

 in animals of distant relationship, and the case in point is probably 

 of this nature. It is to be remembered too, that it is only the division 

 of shell-less Pteropods that in any way simulates the appearance of 

 the Siphonopods; the shell-bearing forms (Thecosomata), such as 

 Creseis, are very closely approximated to the Gastropods in all details 

 of organization. Thus we may conclude that the Pteropods are a 

 branch from the Gastropod stock, modified by pelagic habit, and in 

 some respects even degenerate (i.e. degenerate from the stand-point 

 of the Gastropod) and having their most specialized members approx- 

 imated in outward form to the Siphonopod type. As to this latter 

 designation, it appears thus more fitting to displace it and to restore 

 the term Cephalopoda to its older and more restricted . meaning 

 whereby it is applicable to the Octopus class alone. 



The diagrams PL III, figs. D to H (modified from Lankester) 

 show graphically the mutual relation and modification of the several 

 parts of the body as seen among the principal types of Glossophorous 

 Molluscs. Fig. D shows a simple type of Gastropod, such as Chiton, 

 where the mouth and anus are at opposite ends of the body, the foot 

 large and extending the whole length of the body on the ventral side, 

 while the central part of the back is more or less humped — forming 

 the visceral hump or dome, as in it most of the viscera are lodged. 

 Fig. E indicates the modifications in the relative arrangement of parts 

 due to the bending and coiling of the visceral hump, as seen in such 

 Gastropods as the snail and the whelk. G and F represent respec- 

 tively a shell-bearing and a naked Pteropocl, and show how in these 

 animals the visceral dqme is much elongated and drawn out, causing 

 thus a great bend in the alimentary canal. The foot in both is 

 represented by little else than two wing-like fins, believed to arise 

 from two lateral flaps — epipodia — of the middle division of the foot. 

 In several Gastropods, such flaps are well developed ; thus in the 

 sea-slug Aplysia, they rise from either side of the foot and fold over 

 the back. In F, the " head-arms " or " buccal cones," that are so 

 curiously like the arms of Cephalopods, are represented, but are not 

 here shaded similarly as having a like origin for the reason already 

 given. 



Fig. H is a diagram of a Cephalopod given for comparison. 

 Here part of the epipodia (?) form arms beset with suckers in place 

 of swimming fins, while the funnel is also formed from part of the 



