PHVSIOLOGV OF THE INVERTEBRATA. 187 



The Tkichoscolices. 



'• In the TnrhcUarvi, Trcitiafof/a^ and Ccstoidcf, the lacun:e 

 of the mesoderm and the interstitial fluid of its tissues are 

 the only representatives of a blood-vascular system. It is 

 probable that these communicate directly with the terminal 

 ramifications of the water-vascular (respiratory) system. In 

 the Rotifcra, a spacious perivisceral cavity separates the 

 mesoderm into two layers, the splanchnopleure, which forms 

 the enderon of the alimentary canal, and the somatopleure, 

 which constitutes the enderon of the integument. The ter- 

 minations of the water vessels open into this cavity. 



"* 



The Annelida. 



In the Annelida there is a perivisceral cavity (perienteric 

 space) communicating with the segmental or excretory organs. 

 This cavity contains a colourless fluid consisting of a coagul- 

 able albuminous plasma and numerous colourless corpuscles. 

 The perivisceral fluid is not only nutritive, but acts as a 

 liquid fulcrum to the muscular movements of the body. If 

 this fluid is let out the power of voluntary motion is lost. 

 It has been stated that " the vermicular motions of the 

 intestine are aided or determined by its resistance and 

 support ; it favours circulation by obviating the pressure 

 upon the blood-vessels, which follow the contact of the 

 intestine with the integument, and is, perhaps, the source, 

 or one of the sources, of the blood itself."' This fluid con- 

 tains albumin, flbrin, and certain salts. In addition to the 

 perivisceral cavity and its fluid, there is in most of the 

 Aiindula a system of vessels with contraHilc 'w&\\%. These 

 vessels, known as the pseudo-ha3mal system, are filled with a 

 fluid, which may be red or green, and corpusculated or non- 

 corpusculated. In some Annelids the pseudo-htemal system 

 communicates with the perivisceral cavity ; but in the majority 

 of these animals it is shut off from it. 



* Huxley's Anatomy of Invertehrata, p. 57. 



