438 SANGUIFEROUS SYSTEM. 



trunks, and two shorter veins proceeding outwards from 

 the internal parts. Reinforced by the accession of numerous 

 smaller branches, the two great venous trunks pursue a 

 winding course backwards, receiving also large branches near 

 the junction of the lateral halves of the body, and they can 

 be traced, converging, as far as the opaque muscular organs 

 of attachment, at the posterior ends of the segments, ( 1 32. 

 a. ,) where the two arterial trunks commence. The two 

 great arteries (132. u. v } ) lying more dorsally than the veins, 

 follow a similar tortuous course along the sides of the two 

 segments of the animal, they also are nearly of equal calibre 

 throughout, and their contents are observed to move from 

 behind forwards, while the blood moves backwards in the 

 venous trunks, but neither manifest the slightest systole or 

 diastole. The arteries give off numerous ramifying branches 

 in their whole course, both to the internal and the peripheral 

 parts, those being especially large which proceed to the ute- 

 rine portion, (132. m. n.) of the genital apparatus near the 

 middle of the body, and they become much reduced in 

 bulk, and at length imperceptible, before they reach the 

 sides of the mouth or the commencement of the venous 

 trunks. The; minute anastomosing branches of these great 

 lateral arteries and veins form superficial and deep seated 

 plexuses, which are most obvious in the more transparent 

 parts of the body, and especially a compact subcutaneous 

 plexus spread over the general surface ; but they here form 

 no median dorsal artery, as in most articulated classes, and 

 there appears to be little vascular connection between the 

 two lateral segments of this animal. 



The extensive distribution of the sanguiferous system, the 

 rapid circulation of the blood, and the great superficial 

 plexuses, so conspicuous in the trematode entozoa, buried in 

 the substance of their food, and destitute of anal opening, 

 may enable them to receive or excrete matter through the 

 general surface of their body, as well as through their 

 buccal orifice. This extensive distribution of the circulating 

 vessels has also been especially observed in different species 

 of distoma and amphistoma, where, from the breadth of the 

 body, the principal trunks are commonly double and lateral, 

 and where they have sometimes appeared to communicate 

 directly with the exterior surface, or with internal sacs, or 



