76 HUBERT LYMAN CLARK ON 



such lacunae actually exist. The ventral vessel of the stomach does not lie appressed to 

 its wall, but entirely free from it and connected with it by several small branches. It is 

 also connected by a large transverse vessel with the ventral vessel of the intestine (Fig. 

 92), and the two sections of the latter are also connected by a similar vessel. These 

 transverse vessels do not appear until the animal is several centimeters long, when they 

 arise by outgrowths of the coelomic epithelium of stomach and intestine which, lying 

 close together as they do in the loops of the digestive tract, touch and fuse (Figs. 59 and 

 60) and with the increased growth of the intestine are finally drawn out to slender con- 

 necting vessels. Like the vessels of the young Synapta, these are supposed to be lined 

 with connective tissue, but I have been unable to detect it in their walls. 



The ciliated funnels of Synapta vivipara differ considerably in appearance from those 

 of S. digitata or S. inhaerans, though they do not differ essentially in structure. Only 

 one sort seems to be present and these are quite small but extremely numerous on all 

 three of the mesenteries. They measure from 40/x to 75/t in length, and from 30/x to 

 4(V in breadth and depth, which is only about half the size of those of /S. diyitata. They 

 are broad funnel- or cornucopia-shaped in outline and usually have a short stalk. 

 Their general structure will be easily understood from Figs. 63-65. The water-vascular 

 system consists of a circumoesophageal ring from which canals arise and pass to the ten- 

 tacles, into which their entrance is guarded by well-developed valves. Each tentacle 

 rests on the calcareous ring in such a way that the outer half of the base is on the out- 

 side of the calcareous plate, forming a sort of rudimentary ampulla (Fig. 90). There is 

 not in the adult, any more than in any of the larval stages, the slightest trace of radia. 

 water-canals. Dependent from the ring-canal there is always present in the left dorsal 

 interradius a slender Polian vessel five or six millimeters long, and in nearly all adults 

 additional Polian vessels, sometimes as many as six, are present in the ventral interradii. 

 The stone-canal leaves the water-ring on the left-hand side of the mid-dorsal interradius 

 and does not lie in the dorsal mesentery but clearly separate from, and to the left of it. 

 It soon passes into it, however, on its outward course and runs to the body-wall close 

 beside the genital duct (Figs. 66 and 67). It enters the body-wall on the right of the 

 mesentery and bends upward more or less abruptly, opening finally to the exterior close 

 behind the circle of tentacles (Figs. 66-70). In exceptional cases there are two open- 

 ings (Figs. 71-73) or rarely the reverse happens and the canal closes before the exterior 

 is reached. Besides this external opening, the stone-canal also opens into the body-cavity 

 through a well-developed madrepore (Figs. 66 and 74). Throughout its course the canal 

 is heavily ciliated, and especially so around these madreporitic openings, the whole 

 arrangement being admirably adapted for keeping the body-cavity fluid well aerated. 



