SYNAPTA VIVIPARA. . 65 



exact mid-line. With the closure of the hydrocoel, occurs the union of the two ends of 

 that coelomic tube which lies on its oral surface, so that we now have a circular sinus 

 around the oesophagus just above the water-ring. This sinus is very evident in young 

 Synaptas, and Bury's ('95) surmise regarding its origin from the left coelom is entirely 

 correct. The primary tentacles are growing upAvard, not pushing the floor of the atrium 

 out before them, as Semon ('88) says, but enclosing the atrium within their circle so that 

 the thickened sensory epithelium, which they subsequently possess, does not arise as 

 Semon describes. It is clear, from Figs. 16 and 17, that his description could not possibly 

 appl}' to S. vivipara. The secondary outgrowths remain nearly unchanged in size and 

 show no sio-n of bendino; backward to form radial canals. The Polian vessel is formed as 

 an outgrowth on the inner side of the hydrocoel ring in the left dorsal interradius, as 

 Ludwig ('23) found it to be in Cucumaria. 



The digestive tract grows with greater rapidity relatively than any of the other 

 organs. Accordingly, the oesophagus jjushes upward toward the atrial opening, so that 

 the thickened ectodermal floor of the atrium lies surrounding it, in the form of a poorly 

 defined circnmoral ring. Continued growth pushes the oesophagus against the upper 

 ectodermal wall of the atrium, and with that it fuses, leaving the circumoral ring entirely 

 cut off from the outer ectoderm of the body (Figs. 87 and 88). Meanwhile the growth 

 of the primary tentacles has pushed this body-wall upward and outward, so that the 

 narrow slit-like opening of the atrium is gradually widened until it finally disappears, 

 leaving the ectodermal-covered, anterior end of the oesophagus to form the definitive 

 mouth, in the center of the circle of tentacles. This process is not completed however, 

 until the pentactula form is fully assumed. The differences between this development 

 of the mouth and circumoral ring and that given by Semon ('88) for (S'. digltata are 

 almost irreconcilable, but they are all dependent on the question, whether the five 

 primary tentacles push up through the floor of the atrium or grow up around it. The 

 latter is certainly the case in S. v'mipara. While these changes are taking place anteri- 

 orly, the hind-gut has increased in length so that it has not only arched still more toward 

 the dorsal surface but has bent on itself and formed a loop lying to the left of the 

 stomach. The coelomic pouches already united and forming a single cavity ventrally, 

 have met in, or close to, the mid-dorsal line and by the union of their walls have formed 

 the dorsal mesentery. This mesentery follows pretty closely the curve of the intestine 

 and attaches it throughout its course, to the l)ody-wall. The water-canal lies in the 

 anterior part of the mesentery, but whether that part was formed in a different manner, 

 as Bury ('95) thinks probable, it is impossible to say from observations on S. vivipara. 



Meantime most important changes are going on in the circumoral ring of ectoderm 



