336 Dr. T. Williams on the Mechanism.of Aquatic 



pulsate. Among blood-vessels such a circumstance would con- 

 stitute an irreconcileable anomaly. The blood-vessels described 

 by M. Blanchard, and in common with him by nearly all hel- 

 minthologists, in the Cestoid and Trematoid Entozoa proceed 

 from one or more central sjjaces. When these spaces are cylin- 

 drical in figure they assume the apparent characters of blood- 

 trunks. They really represent the splanchnic or visceral cavity. 

 Such is the form under which this cavity, so conspicuous in the 

 Nematoid Entozoa, occurs in the Sterelmintha. The main cen- 

 tral channels present a genei-al coincidence of disposition with 

 those of the alimentary system (PI. XIII. figs. 3, 6, 8, 9). 

 They do not present a uniform diameter according to the 

 customary manner of a vascular trunk. They exhibit irregular 

 outlines, now contracting into narrow necks, and now dilating 

 into lacunae. This is especially true of those of the Trematoid 

 worms. In the Cestoid orders (fig. 6) the main trunks follow 

 the margin of the " Zoonite," the lesser crossing in the parallel 

 spaces between the transverse annuli into which the integu- 

 ments are wrinkled. These secondary channels in Tania are 

 much more irregular in outline and distribution than the 

 blood-vessels {sic) depicted in the drawings of M. Blanchard : 

 they penetrate intimately the substance of the integuments. 

 With reference to these channels, lai'ge and small, one important 

 fact should be noted : they are not gifted with separate mem- 

 branous parietes. This fact alone is enough to prove that they 

 are not channels for the conveyance of true blood. Every true 

 blood-vessel is endowed with a special power to circulate its fluid 

 contents. Its parietes are contractile. The bore of the channel 

 rythmically increases and decreases. Such movements would be 

 mechanically impossible if the parietes of such ' vessels ' were 

 adherent to the surrounding solid and fixed tissue. In a chan- 

 nel destined to convey chylaqueous fluid the latter case is the 

 rule. They possess no inherent circulating power. Their con- 

 tents are impelled to and fro under the conjoint force of ciliary 

 and muscular action. Here then is a clearly defined distinction 

 between a chylaqueous channel and a true blood-vessel. Let 

 the wonderful vascular system, which M. Blanchard has deli- 

 neated from artificial injections in the Cestoid and Trematoid 

 worms, be tested by this anatomical principle. This excellent 

 observer has omitted to investigate the histological characters of 

 these parts. He has not in any case determined the relation 

 between that system which he describes as the true blood system 

 and the surrounding solid structures. In no instance has he 

 reduced to demonstration the physical characters of the fluid by 

 which his so-called blood-vessels are filled. The same criticism 

 will apply with equal justice to the illustrations, by aid of which 



