52 THE AMPHINEURA 
A gastrula is formed by invagination of the endodermic macromeres 
(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. 
