PHYSIOLOGY OF THE INVERTEBRATA. 187 



The Trichoscolices. 



" In the Turbdlaria, Trematoda, and Cestoidea, the lacunte 

 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 Botifera, 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 favo ursjcirgnlatijQ n by ^obviating the_^ressure 

 up on th e 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, fibrin, and certain salts. In addition to the 

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

 Annelida a system of vessels with contractile walls. These 

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

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

 corpusculated. In some Annelids the pseudo-hsemal 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. 



