258 CEPHALOPODA. 



docs not appear until later, sinking in at the middle of a slight 

 prominence, the anal papilla (Fig. 116 D) ; the posterior part of the 

 intestinal canal, starting from this point, runs towards the mantle, 

 its course being externall}- marked by a slight rising on the surface 

 (Figs. 117 and 118). 



In the embryo of Loligo Pealii, at a stage intermediate between 

 those depicted in Figs. 116 B and C we find, in the oral region, 

 starting from the two angles of the mouth and running first to the 

 optic pits and then passing anteriorly, two rows of cilia; these have 

 been compared by Bkooks with the velum, i.e., with the pre-oral 

 ciliated ring of other Molluscan larvae. We should, in tins case, 

 have to regard the part including the eyes and the very limited 

 region between them as the pre-oral part of the body. 



Among the changes which take place on the dorsal side of the 

 embryo, those that occur in the mantle exercise a special influence 

 on the external form of the animal. The pit representing the shell- 

 gland which, at first, is very wide (Fig. 116, sd) narrows with time, 

 and in later stages is only a small aperture (Fig. 117 B) and finally 

 closes entirely. The external aperture of the continually deepening 

 | lit becomes surrounded by a kind of circumvallation, the surface 

 again sinking in, though not to any great extent (Fig. 116 D). The 

 almost rhomboidal swelling [m) which surrounds this shallow de- 

 pression represents the marginal part of the mantle which now 

 begins to rise above the rest of the bod}'. At a slightly later stage 

 (Fig. 117 A), we find the mantle becoming marked off by the 

 swelling of its margin. The large depression round the circular wall 

 of the shell-gland flattens out again and becomes rounded off like the 

 edge of the mantle itself (Fig. 117 I>). Two pointed prominences, 

 the rudiments of the fins, are visible upon it (Fig. 117 A and />'). 



Another change has taken place in the mantle-region on the 

 dorsal side, the posterior funnel-folds having shifted more towards 

 the middle line, there ending in a kind of plate which is the rudiment 

 of the nuchal cartilage (Fig. 117 IS). The posterior funnel-folds are 

 thus recognisable as the broad muscle-bands (the so-called nuchal 

 muscles) which even in the adult appear as lateral continuations of 

 the funnel. They run to the nuchal cartilage and become attached 

 to it. 



In tracing the further development of the funnel, we find that the 

 anterior folds form the essential factor in determining the alterations 

 that take place in its principal parts. These anterior folds, which 

 soon rise much higher than the posterior folds, become united in the 



