338 Dr. T, Williams on the Mechanism of Aquatic 



blood-vessels [sic) of M. Blanchard terminate in Holostornum ala- 

 tuni, Amphistoma conicum, Tristoma coccineum; in others they form 

 re-entering branches. The tigure of M. Blanchard* represents 

 the secondary trunks in Distomum as proceeding from the pri- 

 mary with far too great regularity. To this trunk he seems to 

 indicate a caudal orifice, and yet calls it a blood-vessel ! It is 

 susceptible of proof that this central channel in Distomum is not 

 contractile. The adjacent trunks of the digestive system contract 

 and dilate in regular periods. The walls of the central channel 

 are perfectly stationary. Its interior is not lined with cilia. 

 Its fluid contents do not move in one systematic orbit : they 

 oscillate to and fro. Such characters can belong only toachyl- 

 aqueous system. 



In Holostornum alatum, Amphistoma conicum, in Tristomata, 

 in Brachyleemus variegutus, sometimes found in the lung of the 

 Frog and Toad, in Monostoma verrucosum, &c., the main primary 

 channels of the fluid system coincide with those of the digestive. 

 The latter are embraced by the former. This anatomical fact is 

 significant of a physiological principle. It points to the manner 

 in which the contents of the chylaqueous passages are derived by 

 exosmosis from the chymous fluid contained in the digestive 

 cseca. In those species of Trematoid worms in which the central 

 conduits are more than one in number, it is important to observe 

 that they are always joined together into one system by inter- 

 vening passages. They do not convey opposite currents. In both 

 the contained fluid flows and ebbs with great irregulainty, in 

 obedience to the contractions and expansions of the muscular 

 integumentary envelope. In these essential particulars the 

 standard definition of a true-blood system is violated. M. 

 Blanchard defines thus the Appareil vasculaire of Distomum he- 

 paticum : — " Get appareil consiste en un vaisseau principal et 

 median." A true circulation requires two primary trunks, an 

 artery and a vein. In another place this helminthologist observes 

 generally with regard to the Trematoda — " Chez tons ces vers, 

 le sang n'est certainement pas transporte, d'une maniere regu- 

 liere, d'arriere en avant par certains vaisseaux, et d'avant en ar- 

 riere par d'autres, comme I'a pense M. Nordmann. Dans les 

 Trematodes en general, le fluide nourricier est transporte et ra- 

 mene alternativement et plus ou moins irregulierement par les 

 memes vaisseaux ; c'est un mouvement de va-et-vient plutot qu'une 

 veritable circulation." (xlnnalesdes Sciences Nat. torn. viii. 1847, 

 p. 336.) Ample evidence is thus drawn from his own obser- 

 vations to convince the physiologist that the system of vessels 



* See plate 36, — Zoophytes : Crochai'd's French eihtion of tlie ' Regne 

 Animal.' 



