342 INVEETEBEATA chap. 



separated off, which differentiates itself into two lateral masses of nerve 

 cells with fibres between them {c.p, Figs. 269, 270, and 271). The 

 pedal and visceral ganglia arise as two pairs of thickenings 'of the 

 ectoderm of the ventral surface, one pair being situated close behind 

 the other (Fig. 269). In addition a pair of pleural ganglia make 

 their appearance as a small pair of thickenings of the lateral ectoderm 

 of the body, half-way between the rudiments of the cerebral and pedal 

 ganglia ; in later life they fuse with the cerebral ganglia (Fig. 274). 



The otocysts arise as small spherical invaginations of ectoderm at 

 the apex of the mantle-groove. They are situated in the region which 

 is afterwards converted into the foot. 



Towards the end of larval life the foot makes its appearance. It 



Fig. 272. — Transverse section through the dorsal region of old Veliger of Dreissensda 



morpha in order to show tlie differentiation of the pericardium and the kidneys. 

 (After Meisenheimer. ) 



ooe, first rudiment of pericardial cavity ; H, heart ; /ua, hinge area of shell ; int, intestine ; Lie, left 

 kidney ; per, ring of cells which gives rise to pericardium ; r.fc, right kidney ; sli, shell. 



is defined by two transverse furrows, an anterior and a posterior, the 

 latter cutting in deeply between the pedal and the visceral ganglia 

 and separating them from one another. On the posterior aspect of 

 the foot a deep invagination occurs which is lined by columnar cells. 

 This is the rudiment of the byssus gland, which secretes the cords of 

 horny material by means of which the adult Dreissensia anchors itself. 

 The forepart of the foot grows into a finger-like process covered with 

 minute cilia, and the primitive kidney disappears. 



. The intestine becomes bent into a slight loop ; it runs upwards 

 from the stomach and bends downwards and forwards to reach the 

 anus. 



The posterior adductor muscle is formed by a modification of 

 some of the spindle-shaped cells of the mesenchyme, and so also is the 

 retractor of the foot. This retractor muscle is a mass of fibres 

 which project downwards from the posterior part of the mid-dorsal 



