150 THE APODOTJS HOLOTHURIANS 



comes a muscle layer, the fibers of which run in all directions parallel to the 

 surface, but the iuuermost are circular and are more numerous than the outer 

 oblique and longitudinal iibers ; then follows a thick connective tissue layer, and 

 lastly an inner epithelium. Presumably the less developed the trees are, the 

 less well developed will the muscle layer be. There are no openings by which 

 the trees can communicate with the body-cavity, but all the branches end 

 blindly. 



Cuviek's organs (Plate X, fig. 10). — The occurrence of these organs in the 

 Molpadiidae is still open to considerable doubt. They have only been reported 

 in a few specimens of Gaudina from Chile, and no account of their finer struc- 

 ture has yet appeared. They are said to consist of a small tuft of spherical 

 bodies of a brown color, somewhat like a bunch of grapes in form, attached to 

 the cloaca near the base of the respiratory trees. That these organs are homol- 

 ogous with the true Cuvier's organs of other holothurians is by no means sure. 



Blood system. — The arrangement of the hmnal lacuna' is very similar to 

 what it is in the Synaptidse, but has been studied carefully only in Caudina 

 arenata. In this species Gerould ('96) recognizes four parts to the system: 

 circular lacuna?, intestinal lacunae, lacuUcB of the reproductive organs, and tentac- 

 ular and radial lacuna^. "The circular lacuna^ which form the center of the 

 system, occupy the connective tissue of the wall of the stomach immediately be- 

 hind the circular canal of the water-vascular system. ' ' They occur in the numer- 

 ous out-growths of the outer layer of connective tissue of the stomach. The 

 ring is therefore diffuse and ill-defined. The intestinal vessels occur, one on the 

 dorsal, the other on the ventral side of the alimentary canal. Gerould does not 

 say whether they are connected with the circular lacunae or not, but as he says 

 they contain numerous blood-corpuscles, while other parts of the system do not, 

 it would appear that there is no direct connection. No statement is made either 

 as to connection between dorsal and ventral vessels, but presumably they are 

 connected by lacunae in the intestinal wall. The ventral vessel on the stomach 

 and first part of small intestine is connected by several cross-branches with 

 that of the second section of the intestine, while at the anterior bend of the in- 

 testine the two parts of the ventral vessel are connected by a delicate sheet of 

 anastomosing vessels. "The two parts of the dorsal intestinal vessel are like- 

 wise connected by anastomosing cross-vessels." The lacuniD of the reproduc- 

 tive organs run longitudinally in the connective tissue of the wall ; no statement 

 is made as to their connections. The radial and tentacular vessels arise from 

 the circular lacunae on the inner or axial side of the tentacle canals and run out- 

 ward into the tentacles; the main vessel bends backward with the radial canal 

 and runs clear to the cloacal opening, lying between the radial water-vascular 

 and hyponeural canals; at the extreme posterior end of the body a circular 

 lacuna surrounds the cloacal opening and unites the radial vessels. None of the 



