168 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
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stream flows to the tip of the process and turns upon itself 
to run backward into the ampulla. When aération becomes poor, 
the tentacles and buccal region become distended with the water- 
vascular fluids, and the posterior part of the body becomes pale and 
contracted; it is probable that at such a time the tentacular valves 
close, and that circulation is confined to the tentacle and the ampulla 
connected with it. 
10. SYSTEM OF HAEMAL LACUNAE, 
In Caudina this system closely resembles that of the Cucumariidae, 
as described by Hamann and Hérouard. It may be considered as 
consisting of four parts: (1) a diffuse ring immediately behind the 
circular canal of the water-vascular system, (2) intestinal vessels, 
(3) lacunae of the reproductive organs, and (4) radial and tentacular 
vessels. 
The cirewar lacunae which form the center of the system, occupy 
the connective tissue of the wall of the stomach immediately behind 
ithe circular canal of the water-vascular system. Arising from the 
external layer of connective tissue of the stomach are extensive out- 
growths of the same tissue covered with peritoneal epithelium ; these 
contain the sinuses which constitute the circular haemal vessel. The 
walls of the sinuses are connected by narrow stalks with the connective- 
tissue layer of the stomach, and distally are united into a nearly con- 
tinuous sheath around the stomach. 
The ventral or antimesenterial intestinal vessel is shown at va. 
sng. m Plate 4, fig. 46. Several cross branches connect the two 
parts of this vessel which run along the descending and ascending 
portions of the small intestine. At the bend between the small and 
large intestines there is a delicate sheet of anastomosing vessels. The 
two parts of the dorsal intestinal vessel are likewise connected by 
anastomosing cross vessels. 
The mesenterial vessels possess a wall composed of peritoneal 
epithelium, continuous with that of the intestine, and beneath this 
muscle fibers, which run longitudinally, the interior being filled with 
loose strands and cells of connective tissue belonging to the deep 
connective-tissue layer of the intestine. The contents, as observed 
in fresh and living material, consist of a colorless plasma, in which 
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