DEVELOPMENT OF CEPHALOPODS l6j 



these nuclei relays of cells carry sensory impulses to the middle-anterior- 

 and posterior-cerebral lobes. The terminal fibres of the axons coming 

 from the retina pass into the plexiform layer of fibres between the two 

 granular layers in the cortical part of the optic lobe ; in this layer they 

 communicate with dendrites of cells in both the superficial and deep 

 granular layers ; while the axons of the granule cells convey the impulse 

 to the central and peduncular nuclei. 



Development of Cephalopods 



The higher Mollusca differ markedly in their development from the 

 lower types, e.g. the univalve shellfishes (Patella, Vermetes, Triton) y 

 Chiton ; Dentalium ; and the bivalve shellfishes such as Pecten. In the 

 higher molluscs there is no trochophore or veliger stage of development. 

 The segmentation is partial : meroblastic — a blastoderm or germinal disc 

 being formed on the surface of a large mass of yolk ; moreover, there are 

 many points in the later stages of development which mark a wide diver- 

 gence of the higher cephalopods from the more simple representatives of the 

 class. 



The trochophore stage is believed to have been completely eliminated 

 during the descent of the higher from the common ancestors of these and of 

 the simpler types ; and also adult features recently acquired by the higher 

 types have been reflected back or impressed on the early stages of develop- 

 ment of the higher types. The explanation of some of these changes 

 probably lies, as has been suggested by MacBride, in the effect which 

 " the accession of large stores of nourishment " in the eggs has in " almost 

 obliterating the traces of ancestral history in their development, leaving 

 only the most general resemblance in the formation of the layers and the 

 development of the sense-organs as links between them and other 

 Mollusca." 



