122 PHYSIOLOGY OF THE INVERTEBRATA. 



Ac.tinozoa. The alimentary canal, or dig-estive system, is sus- 

 pended in the somatic cavity ; and the digested food transudes 

 through the walls of the former into the latter. The nutrient 

 fluid is then absorbed by the walls of the somatic cavity, as 

 well as by the various organs suspended therein. The somatic 

 or peritoneal cavity in the Astcridctc contains a watery cor- 

 pusculated fluid. The corpuscles are nucleated cells ; this 

 fluid therefore represents the blood of the higher animals. 



It will be noticed that in the Actinozoa, as well as in the 

 Echinodcrmatd . the function of absorption is distinct from 

 that of digestion, but it is not performed by any special 

 organs. 



The Cestoidea. 



As already stated, the Cestoidea are reversions to a lower or 

 simpler type. They are immersed in the chyle or the tissues 

 of the higher animals ; consequently they absorb the digested 

 food, &c., by the whole of the external surface. Although 

 these animals are much higher in the zoological scale than 

 the Gregarinida^ there is in the functions of digestion and 

 absorption a close analogy between these two orders. In 

 both, the processes of absorption and digestion are not 

 differentiated. 



The Annelida. 



The digestive tube is suspended in the perivisceral cavity. 

 The digested food transudes into this cavity, and there 

 becomes mixed with a colourless corpusculated fluid. This 

 fluid fills the perivisceral cavity, and is analogous to the blood 

 of other Invertebrates. . This colourless fluid is not contained 

 in any vessels, although there is in Lmiibrietis, for example, a 

 red fluid contained in a well-developed system of vessels, in 

 addition to the colourless fluid already mentioned. 



The nutrient matters, after having passed into the peri- 

 visceral cavity or chambers — as the perivisceral cavity is 

 generally divided into chambers by means of thin muscular 



