THE LIFE-HISTORY OF NUCULA DELPHINODONTA. 385 



seems to be formed by the rearrangement of mesodermal 

 cells. (See pp. 365—367, and figs. 24, 26, 28, 34, 35, 36, 39, 

 41, and 48.) 



Vascular System. — The heart is formed as a mesodermal 

 strand that stretches across the pericardium from one side to 

 the other. There is no indication that it is formed by the 

 fusion of either a pair of pouches or two masses of cells. It 

 seems to be single in its origin. It is formed around the in- 

 testine, but later becomes free and dorsal to it. This seems 

 to show that for this group, at least, the dorsal position of 

 the heart as found in the adult is not primitive. The vas- 

 cular system consists largely of spaces that occur in all parts 

 of the body. (See pp. 367—370, and figs. 41, 48, 67, 68, and 

 69.) 



Nervous System. — The cerebral and pedal ganglia are 

 about equal in size, and the visceral ganglia are considerably 

 smaller. The cerebro-visceral commissures ai'e very large, 

 and contain many scattered nuclei. The cerebro-pedal com- 

 missures show ordinary structure. It is suggested that the 

 smaller, dorsal roots of the cerebro-pedal commissures may 

 be the central ends of the otocystic nerves. (See pp. 870 — 

 374, and figs. 24, 34, 36, 40, 46, and 48.) 



Otocysts. — The otocysts are formed as invaginations from 

 the body-wall soon after the test is shed. They seem to be 

 entirely closed off, but canals connecting them with the 

 surface are present in the adult. The otoliths are irregular 

 bodies, but they are probably formed in the otocysts them- 

 selves. (See pp. 375—377, and figs. 40, 46, 48, and 64, and 

 Text-fig. W.) 



Muscular System, — The muscular system is well devel- 

 oped, and resembles the muscular system of Yoldia in most 

 respects. The extensive attachments of the foot muscles to 

 the dorsal portion of the shell is accounted for by the great 

 development of these muscles. (See pp. 377 — 380.) 



Excretory Organs.— The vacuolated character of the 

 cells of the excretory organs makes it diflBcult to trace some 

 portions of the cavities of these organs. The inner peri- 



