DEVELOPMENT OF THE PECTINIBRANCHI ATA. 347 



delicate part, directed towards the oesophagus, the excretory 

 duct of the gland is indicated, and elongates to meet this por- 

 tion of the intestinal canal (PL XI. figs. 30, 31 g, PL XII. fig. 4). 

 The salivary gland forms but a single conglomerate mass in the 

 adult, but its double excretory duct shows clearly enough, that 

 it was divided into two parts in the very youngest state. 



The heart was seen on the twenty-third day. The mode of 

 its development is analogous to that which occurs in Buccinum. 

 It is situated upon the dorsal side, presents the form of a 

 bladder, and is directed from above downwards and from left to 

 right ; it contracted in this direction, at the rate of forty to fifty 

 pulsations a minute. It possesses primitive muscular fibres, 

 having the form of longitudinal tubes, undivided above. We 

 have met with neither cells nor nuclei in these tubes (PL XII. 

 figs. 1, 3 h). 



In this stage of development, the branchial cavity is not deep 

 enough to contain the whole of the heart, a considerable portion 

 of which extends beyond the edge of the mantle. Subsequently, 

 when the mantle elongates and covers in the back of the animal, 

 its edge is directed more outwards and detaches itself from the 

 body, so that the branchial cavity becomes deeper and wider 

 and encloses the heart completely. 



We have as yet been unable to observe the circulatory current 

 in this mollusk. 



It is only after the formation of these organs that the buccal 

 aperture, the proboscis and the oesophagus, are perceptible. 

 The proboscis is exceedingly short and its parietes are very 

 thick, so that it is easily seen through the oesophagus (fig. 3H). 

 The latter is cylindrical and takes a course beneath the stomach 

 (figs. 31, 32 k). The latter lies to the left, it is small and oval, 

 and a long and delicate intestine passes out of it, which turns 

 to the right, bends back afterwards to the opposite side in a 

 curved direction, and finally terminates in the branchial cavity 

 by a projecting anus (fig. 32 /, m, n). 



The oesophagus, the stomach and the intestine are ciliated 

 upon their inner surface. 



It is not until a somewhat later stage of development that the 

 nervous system is distinctly discoverable. It is composed of 

 two cerebroid ganglia upon each side of the oesophagus (figs. 31 n, 



