DR. CARPENTER, ON COMPARATIVE PHYSIOLOGY. 217 



occasionally to be observed about the head (as in Nemertes) particular 

 groups of cilia longer and more closely set than the rest, reminding us of 

 the ' wheels ' of the Rotifera. 



This ' aquiferous ' system of vessels presents its greatest complexity and 

 most elevated character in the group of Entozoa; whilst at the same time, 

 there are certain types even of that class, in which it exists mider its most 

 simple and least doubtful aspect. It is only, in fact, by tracing it through 

 its principal fonns and gradations, that the real import of some parts of this 

 system can be ascertained. In the Cestoid worms, we find four principal 

 canals, two on each side (fig. 1, a h), running along the body at or near 



Fig. 1. 



the margins of the segments, and connected toge- 

 ther by transverse branches ; these anastomose 

 with one another freely in the head, those of the 

 opposite sides being generally connected by an 

 arched canal ; whilst at the opposite extremity 

 they all terminate in a single contractile sac opening 

 externally. Besides these trunks (of which the 

 two larger, a, have been considered as a double 

 alimentary canal), there is a superficial system 

 of vessels more minutely distributed, which has 

 been regarded as sanguiferous ; but these may be 

 traced into connection with the preceding, and 

 their parietes are furnished with vibratile cilia, 

 which keep up a movement of their contents. 

 There is, then, no true sanguiferous system in 

 the Cestoid Worms, any more than there is an 

 alimentary canal ; both being replaced by the di- 

 rect absorption of the nutritious juices from with- 

 out, in a state ready for assimilation. Yet it is 

 probable that, as in the cases last cited, the 

 ' water-vascular ' system contains some other fluid 

 than pure water ; and it may even serve, as Pro- 

 fessor Van Beneden has suggested, for a urinary 

 apparatus. The fluid contents of these vessels, 

 whatever their nature may be, are kept in motion 

 partly by ciliary action in their interior, and partly 

 by the contractility of their walls ; the former method seems to prevail in 

 the smaller trunks, the latter in the large. — In the Trematode Worms, we 

 find a regular gradation from what is unquestionably a ' water-vascular,' 

 system homologous with that of the Cestoidea, to an apparatus which 

 nearly approaches the reputed sanguiferous system of the Annelida. One 

 of its most interesting forms is that presented by the Distoma teretkoUe ; 

 in which we find an elongated contractile canal, terminating posteriorly in 

 an external orifice, giving off two contractile trunks, which pass along the 

 two sides of the body without any change of dimension, and unite in an 

 arch above the mouth. The fluid of these vessels is colourless, and 

 contains some minute corpuscles. But besides these, there are a consider- 

 able number of smaller trunks, giving-off branches that form a net-work 

 resembling that shown at fig. 2 ; these trunks, however, have been dis- 

 tinctly found by M. Van Beneden to discharge themselves into the two 

 principal lateral canals, so that they nmst be considered as forming one 

 system with them, although the fluid which they contain is coloured. 

 This system of vessels, therefore, although usually described as sanguiferous, 

 cannot be properly regarded in that light ; and it is obvious, that even 

 when (as happens in the Echinorhynci, which are closely allied to the Tre- 

 matoda) there are no pale, ciliated, non- contractile aquiferous vessels, ter- 



Segment of Tania Solnini. 

 a b, the larger and smaUer 

 longitudinal trunks ; c, 

 ovary ; d, genital orifice. 



