1901,] NATURAL SCIENCES OF PHILADELPHIA. 699 



and the rhyuchocwl, so that only the triangular middle part of 

 the median vessel now remains. 



Immediately behind the ventral brain commissure the unpaired 

 dorsal blood vessel arises from the median vessel, figs. 16, 21, D. V. 

 It passes up through the muscle layers of the proboscis sheath into 

 the rhynehoccel, and continues in this position — on the floor of the 

 rhynchoca?l in the median line, but bounded dorsally by the rhyn- 

 chocoilomic epithelium — until about the middle of the nephridia} 

 region, fig. 22. Here the dorsal vessel passes down through the 

 proboscis sheath and out again into the tissue that is just beneath 

 the rhynchociel and above the alimentary canal; and this position 

 is retained throughout the remainder of its course, figs. 23, 24. 



In ihe anterior tesophageal region the lateral blood vessels are 

 united beneath the oesophagus by ventral connectives. In one speci- 

 men the connectives are broad and dilated, fig. 22, V.hl.con., in 

 others they are almost entirely shrunken together. The ventral con- 

 nection of the lateral trunks is continued throughout the ffisopha- 

 geal region, forming a network of fine anastomosing branches. It 

 is probably further continued, in the remainder of the body, but 

 I have been unable to follow it in my specimens. 



In the oesophageal region the lateral trunks he on the dorso-lateral 

 side of the alimentary canal; in the middle intestinal region they 

 have moved ventrally and lie on the ventral side of the intestine. 



No connection between the dorsal vessel and the lateral trunks 

 has been observed, except at the extreme posterior end of the 

 body. Some little distance in front of the end of the body 

 proper (see fig. 17) the dorsal vessel divides into two, which for a 

 time lie side by side, their walls adjoining. They then move apart, 

 and lie in about the same relative distance from one another as do 

 the two ventral lateral trunks, so that in a section of this region, 

 fig. 26, four vessels ai-e present, symmetrically placed. 



Just anterior to the anal opening the two dorsal vessels, or rather 

 the dorso-lateral vessels, descend and unite with their ventral fellows 

 in a large blood lacuna, fig. 27, Bl.L. This lacuna now occupies 

 most of the area inside the body musculature, for the intestine curves 

 dorsally and shortly opens to the exterior on the dorsal surface 

 of the body. The large lacuna passes on into the caudicle, fig. 

 17, Bl.L., and continues throughout its length. It has no definite 

 walls, but is bounded bv the muscular wall of the caudicle, on the 



