THE LAMELL1BRANCHIA 



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 Yoldia (Fig. 225, a.c), the Mytilidae, Dreissensia (Fig. 224, /), 

 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 



Fio. 224. 



Veliger of Dreissensia polymorpha, left-side view, n, anus ; a.a, anterior adductor ; by, 

 byssus gland ; cr, caecum of the crystalline style ; /, foot ; /, flagellum ; h, heart ; t, intestine ; 

 I, liver ; m, mouth ; mu, retractor muscles ; ot, otocyst and pedal ganglion ; p.a.c, post-anal 

 cilia ; re, embryonic kidney ; .s, 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 bycsus 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, ot). 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 



