XII PHYLUM MOLLUSCA 803 



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 neighbour- 

 hood of the mouth in some forms, perhaps represent the velum 

 orpre-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 second pair — the prehensile arms 

 {ar. 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, etc.) 

 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 b}' a long way 

 the largest of invertebrate animals. Like the other classes of 

 Mollusca they are most abundant in tropical and warm-temperate 

 seas. 



If the Ammonites are to be included among the Tetrabran- 

 chiata, that sub-class was most abundantly represented during 

 the Mesozoic period. The nautiloid Tetrabranchiata were 

 most abundant in the Paleeozoic epoch, during which there 

 lived a great variety of forms of this group, the shell 

 being straight {Ortlioceras), or curved {Phragmoceras), or in a flat 

 spiral with the turns not in contact, or in a helix, or a flat close 

 spiral (Nautilus and others). The earliest representatives of the 

 Nautiloids are found in rocks of Cambrian age ; they are com- 

 parativelj^ scarce in the Mesozoic epoch and in the Tertiary, 

 and are represented at the present day only by the genus 



