Chap, x.] DIGESTIVE APPARATUS IN THE ANIMAL SERIES. 159 



It is thus that digestion is effected in the infusoria, very common 

 in vegetal infusions, — the paramaecia. (Fig. 5.) 



In young sponges, there is a permanent cavity which is also 

 the general cavity of the body. This cavity, which at first had 

 no special covering, clothes itself afterwards with 

 vibratile cilia, and ramifies into anastomosed canals 

 by which circulate the alimentary matters. It is, 

 as we see, a rude model of digestive system, and 

 of circulatory system, which are still conf oimded 

 (nardon). In other sponges there are a mouth, a 

 canal and fine ramifications debouching on the 

 surface of the animal by smaller orifices (sycon, 

 &c). 



Gegenbaur, whom we chiefly follow in this 

 description,^ places after the sponges the inferior 

 animals, to which he and Haeckel give the name 

 of cselenterates (icotXoe, tvTepov). The morphological 

 characteristic of these animals is a body with 

 constant cavity, which may be considered a 

 digestive cavity, sometimes ill differentiated, how- 

 ever ; for the freshwater polypus, for example, if 

 we turn it inside-out, after the fashion of a glove-finger, and 

 keep it thus by means of a thread, can digest with what was 

 previously its exterior surface. Nevertheless in many hydrarians 

 we already see developed the division of the digestive tube into 

 three parts, an cesophagian portion, a dilated portion, and a 

 narrowed portion, terminating in a caecum. 



The organised beings live as they can, become what circum- 

 stances permit them to become, and all processes are good to 

 what we call nature, provided they attain their object. Thus the 

 hydrarian polypi living in colonies, have an intestinal tube in 

 common, prolonging itself through the whole tribe, and realising 

 thus the most perfect communism. 



In the colonies of Siphonophores the specialisation has taken a 

 1 Gegenbaur, Manuel d' Anatomic Oomparie. 



Fig. 5. 

 Schema of the di- 

 gestive cavity in 



a, cavity of the 

 body filled with 

 soft protoplasm ; 

 i, huccal open- 

 ing ; c, anna ; 

 d, d, contractile 

 cavities. 



