PHYSIOLOGY OF THE INVERTEBRATA. 127 



pupae becomes black : and Dr. C. A. MacMunn * has shown 

 that the blood of Helw pomatia assumes a blue tinge on ex- 

 posure to air. 



Concerning the composition and nature of the Invertebrate 

 blood generally, further remarks will be given later in this 

 chapter. 



The Protozoa and Porifera. 



These animals are without blood, for no part of the sarcode 

 can be regarded as blood. The sarcodic substance lining the 

 canals, which traverse the skeleton of the Porifera, is also 

 devoid of any fluid analogous to the blood of the higher 

 Invertebrata. 



In some of the Cestoidea and allied forms the blood or 

 nutritive fluid found " in those interstices of the mesoderm 

 that represent the somatic cavity of other animals, is said to 

 be free from corpuscles." The simplest form of Invertebrate 

 blood is present in the Nematoiclea. 



In the Polyzoa the fluid contained in the perivisceral cavity 

 consists largely of water, and has but few, if any, corpuscles. 

 This nutritive fluid (the chylaqueous fluid of some writers), 

 derived in the first instance from the food that has been 

 digested in the alimentary canal, and which has transuded 

 through the walls of that canal, is, without doubt, analogous 

 to the blood of higher forms. 



In the Hydrozoa, which are provided with blood, the blood 

 is of a very watery nature. The amount of fibrin is extremely 

 small ; consequently the fluid is non-coagulable, and it is 

 almost devoid of corpuscles. That the so-called chylaqueous 

 fluid is analogous to the blood of higher forms is demonstrated 

 by the fact that the perivisceral fluid of the Annelida yields 

 on investigation " not only albumin and fibrin, but crystals 

 which are derived from the water that constitutes so large a 

 part of the nutritive fluid." 



From the above remarks it will be observed that the blood 

 * Quarterly Journal of Microscopical Science, i?iZt,. 



