THE LAMELLIBRANCHIA 247 
a ciliated border, lying in front of the blastopore. It is never 
lobed, and is often provided with a single central flagellum, e.g. 
in YVoldia (Fig. 225, a.c), the Mytilidae, Dreissensia (Fig. 224, fl), 
Cardium, Montacuta, Entovalva, Mactra, and Teredo, but there is no 
flagellum in Pecten, Ostraea, Avicula, Pholas, Nucula, and the 
Unionidae ; it is very much reduced in the incubatory species and 
quite nul in Pisidium. As will be explained further on, the velum 
may be turned back in such a manner as to cover and adhere to 
the whole body of the larva, thus giving rise to the ‘testaceous ” 
larvae peculiar to the Nuculidae. (3) In almost all Lamellibranchs 
an important invagination is formed near the posterior extremity 
of the foot: this is the byssogenous cavity (Fig. 224, by), which is 
ee 
Fia. 224. 
Veliger of Dreissensia polymorpha, left-side view. a, anus; a.a, anterior adductor; by, 
byssus gland ; er, caecum of the crystalline style ; f, foot; jl, flagellum ; h, heart; 7, intestine ; 
1, liver; m, mouth; mu, retractor muscles ; of, otocyst and pedal ganglion; p.a.c, post-anal 
cilia; re, embryonic kidney; st, stomach; v, velum; v.g, visceral ganglion. (After 
Meisenheimer.) 
formed even in species devoid of a byssus in the adult state. In 
Cyclas, for instance, there is a larval byssus by which the embryo 
attaches itself to the incubatory branchial cavity. (4) In the middle 
of the velar area there is an apical plate formed by an ectodermic 
thickening, from which the paired cerebral ganglia originate. In 
Yoldia each cerebral ganglion is formed from a deep tubular 
invagination, and similarly in Dreissensia an apical fossa grows in 
from the apical plate, and the cerebral centres are formed from 
its deeper part. The pedal centres arise from ectodermic 
thickenings between the larval mouth and anus (Fig. 224, of). The 
pleural ganglia are distinct from the cerebral during larval life, in 
Dreissensia, Modiolaria, Lasaea, and Teredo, at any rate. Two larval 
eyes with cuticular lenses occur in many forms, on either side the 
