254 EMBRYOLOGY OF TEREBRATULINA. 



color. In the course of a few hours they become clothed with cilia, and while many 

 of them slowly move away, some remain and perish, or, at least, show no sign of devel- 

 opment. These are probably not fertilized. 



The sexes are separate, and the spermatozoa are discharged by the males in the same 

 way that the eggs are discharged by the female, but whether the eggs are fertilized after 

 they have left the parent, or before, I was not able to determine, as different lots of eggs 

 behaved differently after their escape. In some cases the eggs did not appear to be locomo- 

 tive until two or three days after their discharge, while in other cases they became active 

 on the day of their discharge. 



Great care must be taken to change the water every day, since the water soon becomes 

 vitiated, and Paramjecia rapidly develope and appear to feed upon the embryos. The tem- 

 perature of the water must be kept as near as possible to that from which they were 

 taken. In drawing and replacing the water a glass syphon was found the best, as in this 

 way the embryos were not disturbed, though some skill was required during the active 

 natatory condition of the embryos, to prevent them from leaving the dish also. 



The development of the embryo presents a series of well defined stages, and I shall 

 consider each stage in turn. In the first stage the embryo becomes widened at one end. 

 The segments are barely indicated, the .posterior end is the widest, the anterior portion is 

 ornamented with a conspicuous tuft of long cilia, so peculiar to the embryos of many 

 worms. The embryo is also clothed with vibratile cilia, and in this condition slowly 

 moves along the bottom of the dish without rising from it, or remains quiet. In the sec- 

 ond well marked stage the embryo is divided into two prominent segments, these expand 

 and contract upon each other slightly, and the cephalic segment has the power of partially 

 bending from side to side. In this stage the embryo is most active, swimming rapidly in 

 every direction and turning abruptly about. The oesophagus also becomes dimly denned. 

 In the third stage the peduncular segment is developed and projects from the posterior 

 portion of what can now be called the thoracic segment. At this stage the embryo either 

 remains immovable upon the bottom of the dish or slowly moves about. In two cases deli- 

 cately barbed seta? to the number of thirty-five projected directly backward from the pedun- 

 cular segment. In the fourth stage the embryo becomes attached by means of its peduncular 

 segment. The embryo is still clothed with cilia, though the long pencil of cilia has disap- 

 peared. The head is closely drawn to the thoracic segment, which becomes wider in trans- 

 verse diameter, so as nearly to hide the peduncle. In the fifth stage the thoracic ring 

 commences to fold, or turn upward upon opposite surfaces of its circumference, so as to 

 gradually enclose the head ; one fold being made slightly in advance of the other represents 

 the larger or ventral valve. In this stage appear clusters of barbed and deciduous setos 

 upon the anterior margin, and in a later portion of this stage the first hardened areas of 

 the dorsal and ventral plates make their appearance, and the cirri appear as blunted papil- 

 lae about the mouth. In the sixth stage the shell becomes rounded, the peculiar scaled 

 structure makes its appearance, and the formation of tubules perforatiiig the shell, and per- 

 manent setae takes place. 



In another memoir I hope to present the characters of the genitalia of the Brachio- 

 poda. I will state here, however, that the eggs not only fill the large pallial sinuses, but 

 hang in clusters from the genital band ; from these parts they escape by dehiscence, and 

 float freely in the perivisceral cavity. The anterior perivisceral wall is sufficiently trans- 



