52 THE AMPHINEURA 



A gastrula is formed by invagination of the endodermic raacromeres 

 (Fig. 10, A). The orifice of invagination or blastopore is placed 

 on the vegetative pole of the ovum, and does not close, but is 

 gradually shifted towards the anterior end of the embryo. By the 

 development of a ciliated ring or velum, in the centre of which 

 there is an apical tuft of cilia, the embryo becomes a trochosphere 

 larva (Fig. 14, A). The mesoderm arises from two endodermic. 

 cells, originally situated at the hinder side of the blastopore : they 

 give origin to two bands of cells which extend right and left between 

 the enteron and the ectoderm. In each band a cavity appears 

 which becomes half of the coelom. The ectoderm around the 

 blastopore is invaginated to form the oesophagus, a diverticulum 

 of which becomes the radular caecum. At a late stage of develop- 

 ment the intestine is placed in communication with the exterior 

 by an anal invagination or proctodaeum. Anteriorly on the ventral 

 or pedal surface a second ectodermic invagination forms the great 

 pedal mucous gland, which is more or less atrophied at a later 

 period. The gills appear as papillae much later than the anal 

 invagination, the posterior one the first. The gonads originate by 

 proliferation of the anterior wall of the pericardium. 



The four great nerve-cords originate as four longitudinal and 

 parallel ventral thickenings of the interior surface of the ectoderm, 

 the two thickenings on each side of the body being closely approxi- 

 mated to one another at the time of their first appearance. At 

 the extreme anterior end of each lateral or pallial cord and behind 

 the velum is an eye, containing a closed cavity, or forming a 

 simple epithelial ocellus. This eye disappears at a later stage, but 

 in some species persists for a considerable length of time. 



The shell-valves are formed as transverse thickenings of the 

 dorsal cuticle behind the velum, the tegmentum being the first 

 part of each valve to be laid down. The eight valves generally 

 make their appearance simultaneously (Chiton olivaceus, Ischnochiton 

 magdalenensis), but sometimes the eighth valve is formed later than 

 the others (Chiton polii). 



IV. BIONOMICS AND DISTRIBUTION. 



The Polyplacophora are marine animals of sluggish habit, 

 creeping slowly and for no great distance. They are generally 

 herbivorous, and the majority of species live in the littoral zone, on 

 rocks or under stones, but some inhabit the deeper regions of the 

 sea, extending to a depth of more than 2100 fathoms (some 

 Lepidopleurus, etc.). They can easily be kept in aquaria. Poly- 

 placophora are distributed in all oceans and seas ; more than three 

 hundred living species have been recorded. They are fossil from 

 the Ordovician. 



