1897. J NATURAL SCIENCES OF PHILADELPHIA. 155 



For the movement of the branchise there are well developed mus- 

 cle groups. In the free swimming larva, and also in the adult form 

 there are groups of muscles for the movement of setse. 



The egg-chain found in the dorsal furrow consists of from one to 

 four rows, each containing from ten to fifteen eggs. The color of 

 these eggs varies from brown to orange according to the stage of 

 development. Each egg is enclosed in a capsule, while all the ova 

 lie in a common membranous sac. The eggs are telolecithal with 

 considerable nutritive yolk, as the larva does not leave the egg cap- 

 sule until far developed. 



The shell in its earliest formation is trumpet-shaped : later it 

 gradually assumes the tightly coiled spiral form. As in all fixed 

 forms, the body characters undergo considerable modification dur- 

 ing development, and these are greatly influenced by the develop- 

 ment of a limy covering. The shell gland is placed in the ante- 

 rior-thoracic region, in the median ventral line. 



S. borealis is hermaphroditic, the generative products arising in 

 the walls of the perivisceral body cavity. The reproductive glands 

 are arranged on either side of the intestinal canal near the stomach. 

 Where the ova or sperm is developed is distinguished merely by 

 the presence of the products. The eggs pass into the body cavity 

 and from here into the operculum, where they are fertilized and a 

 capsule secreted ; from here they pass out through the opening of 

 the operculum and are placed in the mid-dorsal furrow. The oper- 

 culum of S. borealis does not serve for a brood-pouch as does that 

 of S. spirillum. 



DEVELOPMENT. 



The unfertilized eggs are much smaller in size, have a paler color, 

 and have a much more prominent nucleus than the fertilized eggs. 

 The unsegmented egg has a brown color, the yolk is evenly distrib- 

 uted in large and small globules ; the nucleus is very small. 



The following table will serve to illustrate the usual length of 

 time occupied by segmentation : — 



