262 



CEPHALOPODA. 



follows it, a distinction which is retained in later stages. This pair 

 of arms, the prehensile arms, is soon followed by a third pair (Fig. 

 116 C and D). When the first three pairs have become distinctly 

 differentiated as button-like prominences, the other two which lie 

 nearest the mouth are still mere transverse swellings : the first of 



— met 



an 



a 



-■®mL 



w 



/ 



\ 



ds 



Fig. 120. -Two older embryos of Loligo vulgaris, .1, seen from the tunnel side, B' 

 from the oral side (original). ",-"i- arms; aw, eyes; ds, yolk-sac ; ft, tins; km, 

 nuchal muscle ; ma, mantle; rt, retractor of the funnel (tr). In .1, the gills project 

 below the mantle : between them is the anal papilla. 



these to become distinct is the fourth, then follows the fifth.* As 

 the embryo continues to develop, the arms grow in length, and the 

 sucking discs appear on them. The change of shape undergone by 



* In speaking here of the first to fifth pairs of arms, they are numbered 

 according to the order in which they originate in the embryo, not according 

 to the order sometimes adopted (for which we can see no good reason) in 

 which, on the contrary, the pair of arms which lies furthest dorsally counts 

 as the first and the most ventral pair as the fourth, the prehensile arms being 

 reckoned separately, evidently on account of their different development, 

 which seemed to give them a claim to a special position. 



