vii MOLLUSCA ONTOGENY 253 



layer, while its inner wall, which is applied to the intestine, forms the visceral 

 layer of the mesoderm. The cells of the mesoderm soon become detached from one 

 another (Fig. 209, D) ; they assume the spindle shape and finally fill the segmenta- 

 tion cavity like a network. 



In the meantime the velum has appeared, and, between it and the anus, the shell- 

 gland forms. The cesophagus arises as an invagination of the ectoderm, which soon 

 becomes connected with the midgut. By the addition of a paired primitive kidney, 

 the typical Molluscan Trocophora is formed ; this at first is quite symmetrical, 

 the anus lying posteriorly in the middle line. 



After the development of the cesophagus, a mass of mesoderm cells collects on 



each side of and below the intestine, this mass soon becoming hollow. In this way 



two mesodermal sacs are formed which approximate towards the middle line till 



they touch, and then fuse to form one sac, the double origin of which is still, for a 



time, evidenced by the presence of a median septum. The sac which thus arises is 



the pericardium. Fig. 210 A shows a somewhat further developed embryo seen 



from the right side. Below and behind the mouth are seen the projecting rudiment 



of the foot, on which to the right and left the auditory vesicles have arisen as 



invaginations of the ectoderm. In the pretrochal area, protuberances to right and 



left represent the rudiments of the tentacles, at the bases of which the eyes have 



appeared as ectodermal pits. The shell gland has secreted a shell. The greater 



growth of that side of the body which is covered by the shell has caused a bending 



by which the anus is shifted towards the ventral side. Immediately behind the 



anus, the ectoderm bulges out to form the rudiment of the mantle fold, so that the 



anus comes to lie in a shallow depression, the rudiment of the pallial or respiratory 



cavity. It is important to note that at this outwardly symmetrical stage, the mantle 



cavity and the anus lie posteriorly. The fore-gut (oesophagus) has greatly lengthened. 



The digestive gland has grown out from the stomach ventrally in the form of a 



wide sac, but is still connected with the latter by a wide aperture. The pericardium, 



in which the septum is still visible, has already somewhat shifted from below the 



stomach to its right side. The rudiments of the definite nephridia next form in 



the following way (Fig. 210, D). In each division of the pericardium (the left 



division being smaller than the right) the wall bulges out ; the right outgrowth 



becomes the secreting portion of the permanent kidney ; the left degenerates, but 



must be regarded as a temporarily appearing rudiment of the (original) left kidney. 



The mantle cavity, which lies beneath the pericardium, presses into it to the right 



and left in the form of two projections. The right projection, continuing to grow, 



becomes connected with the rudiment of the right kidney and forms its efferent duct. 



The left projection does not grow further, nor does it become connected with the 



rudiment of the left kidney. 



A further stage is depicted from the right side in Fig. 210 B. The following are 

 the most important alterations. The optic pit has become constricted into an optic 

 vesicle. The mantle fold has grown further forward, and has become deeper to the 

 right. The undivided pericardium has shifted altogether to the right of the 

 stomach, and lies above the rectum, which bends forward and downward. The 

 body is already asymmetrical. 



At the following stage (Fig. 210, C) the posterior and dorsal region of the body 

 rises distinctly from the rest as a visceral dome ; the shell covering this part of the 

 body has increased considerably in size. The mantle fold has become much broader, 

 and the mantle cavity much deeper ; the latter now lies chiefly on the right side of 

 the body. The looping of the intestine is far more marked. On the posterior and 

 dorsal side of the pericardium, the pericardial wall sinks in the form of a channel, 

 which soon closes and forms a tube ; this is the rudiment of the heart. The two 

 apertures of the tube, where the wall of the heart passes into that of the pericardium, 



