_Development._--The eggs may be laid separately invested by a
chitinous envelope, or as in _Ischnochiton magdalenensis_ they may
form strings containing nearly 200,000 eggs, or the ova may be
retained in the pallial groove and undergo development there, as in
_Chiton polii_ and _Hemiarthrum setulosum_. One species
_Callistochiton viviparus_ is viviparous and its ova develop without a
larval stage in the maternal oviduct. Segmentation is total and at
first regular, and is followed by invagination, the blastopore passing
to the position of the future mouth. By the development of a ciliated
ring just in front of the mouth the embryo becomes a trochosphere. In
the centre of the praeoral lobe is a tuft of cilia. Just behind the
ciliated ring is a pair of larval eyes which disappear in the adult;
these correspond to the cephalic eyes of Lamellibranchs. An ectodemic
invagination forms a large mucous gland on the foot, which is more or
less atrophied in adult life. The gonads originate by proliferation of
the anterior wall of the pericardium. The shell-valves arise as
transverse thickenings of the dorsal cuticle behind the ciliated ring,
the tegmentum being the first part formed.

_Classification_.

[Illustration: After Hubrecht, loc. cit.

FIG. 7.--Diagrams of the nervous system of Amphineura.

A, Proneomenia.
B, Neomenia.
C, Chaetoderma.
D, Chiton.
c, Cerebral ganglia.
s, Sublingual ganglia.
v, Pedal (ventral) nerve-cord.
l, Visceral (lateral) nerve-cord.
pc. Post-anal junction of the visceral nerve-cords.]

[Illustration: From Gegenbaur, _Elements of Comp. Anatomy._

FIG. 8.--Anterior part of the nervous system of _Chiton cinereus_,
in more detail.

B, Buccal ganglia (concerned with the odontophore).
C, Cerebral nerve-mass.
P, Pedal ganglion and commencement of pedal nerve-cord.
pl, Visceral nerve-cord. The sublingual ganglia are not lettered.]

Suborder I. EOPLACOPHORA, Pilsbry.--Tegmentum coextensive with
articulamentum, or the latter projecting in smooth unslit plates.

Fam. 1. _Lepidopleuridae._--Terminal margins of end valves never
elevated; form oval or oblong. _Lepidopleurus cancellatus_, Sow.
North Atlantic and Mediterranean; various abyssal species. _Hanleya
hanleyi_, Bean, north Atlantic. _Hemiarthrum Microplax_. The extinct
_Gryptochitonidae_, Pilsbry, with other Palaeozoic ge