368 



R. T. GUNTHER. 



slieath surrounding the viscera is an adaptation for swimming- 

 similar to that in Phyllirhoe, whose ancestors, no doubt, pos- 

 sessed both foot and shell. The subdivision of the muscle 



.Long. muse. ^ 



Text-fig. 4. — Transverse sections showing musculature. 

 A. Cbfetognath (after Hertwig). b. CliEetoderma (after von GrafiF). 



bands into four quadrants finds an exact counterpart in 

 Cha)toderma (von Graff, text-fig. 4). The homology of these 



Text-fig. 5. — Endocepbalic skeletons. 

 A. Spadella marioni (after Gourrei). u. Nautilus (after Lankester). 



muscles with the four longitudinal retractor muscles of the 

 head of the Cephalopoda will be noticed later. 



