Xn PHYLUM MOLLUSCA 779 



forwards. Behind the anterior funnel-folds appear two pit-like 

 depressions, which subsequently develop into the statocysts. 



The elevations on which the eyes (eye) are situated become 

 more and more prominent. The eyes themselves are formed 

 from a part only of these elevations ; each is a pit which sub- 

 sequently becomes closed to form a vesicle the optic vesicle : 

 later an ingrowth of the ectoderm over this gives rise to the 

 lens. 



The embryo covers only a part of the egg, and as it develops, it 

 withdraws itself more towards the animal pole, at which the 

 germinal disc was originally situated a constriction, which soon 

 becomes very deep, separating it off from the rest of the egg ; the 

 latter, consisting of the greater part of the yolk enclosed in a thin 

 layer of blastoderm, forms a rounded appendage of the embryo 

 the yolk-sac (yk. s.}. The yolk-sac undergoes contractions, which 

 are due to the action of contractile cells in the thin mesoderm 

 lining it, and by this means the yolk is forced into the interior of 

 the body of the embryo. 



The anus appears as an aperture situated on a Little papilla 

 the anal papilla. A. row of cilia, which are developed in the 

 neighbourhood of the mouth in some forms, perhaps represent the 

 velum or pre-oral circlet of other molluscan embryos. The mantle 

 now increases in extent, and its margins become more prominent. 

 The anterior funnel-folds grow out and unite in the middle 

 line ; and these, with the posterior folds, go to form the 

 completed funnel together with the " neck-muscles." For a time 

 the edges of the two folds which form the funnel remain free ; 

 eventually they coalesce into a complete tube. 



The edges of the mantle grow out into prominent folds to form 

 the mantle-cavity, into which the gills are drawn. Lateral out- 

 growths have already given rise to the rudiments of the fins. The 

 arms grow out into more and more prominent processes on which 

 the suckers are developed, the fourth pair the prehensile arms 

 (aj. 2) soon becoming distinguishable from the rest by their 

 greater length. 



As the embryo increases in size, the yolk is gradually absorbed, 

 and the yolk-sac decreases in bulk, until, when the embryo leaves 

 the egg, it has almost completely disappeared. 



Distribution. The Cephalopoda are all marine, and range from 

 tidal limits to a considerable depth. A large number (Loligo, &c.) 

 are pelagic and move together in great shoals. Sepia lives chiefly 

 between stones and in rock-fissures in the littoral zone, and often 

 burrows in sand. Octopus constructs a den or shelter of stones to 

 which it always returns after excursions in search of food. Cephalo- 

 pods are, nearly without exception, carnivorous. In length they 

 range from an inch or two to as much as fifty feet the gigantic 

 members of the group, such as Architeuthis, being by a long way 



