CEPHALOPODA 



599 



filament is protruded. In Philonexis and Argonauta the modified arm 

 is charged with spermatozoa, inserted into the mantle cavity of the 

 female and then detached. This arm was described by early observers 

 as a parasitic worm and named Hectocotylus . 



4d.. 



cor. 



cor. 



pig.cp. 



op.n. 



Fig. 409. Eyes of Mollusca. A, Nautilus. B, Helix. C, Sepia. D, Pecten 

 (inverted type), cil.m. ciliary muscle; cor. cornea; d.ret. distal and pr.ret. 

 proximal layers of the double retina of Pecten ; ir. iris ; /. lens ; Id. eyelids ; 

 op.g.y op.n. optic ganglion and nerve; pig. ep. pigmented epithelium ; ret. retina ; 

 tap. tapetum ; vit.h. vitreous humour. In Sepia the cartilage is shown in black. 



Other Dibranchiata. The members of this group are classified in 

 two suborders, whose members respectively possess, like Sepia, ten 

 arms (Decapoda), or, like Octopus, only eight (Octopoda). In no 

 member of either division is there any known form in which the shell 

 is external ; in all cases the shell is more or less rudimentary or, in the 

 case of the Octopoda, entirely absent. There is a well-known and 



