ALIMENTARY CANAL OF C(ELENTEEATA. 



Ill 



known; just as uncertain is that of the two ciliated surfaces, on 

 either side of this vesicle — the polar areas, which are surrounded 

 by short fringe-like processes. 



Alimentary Canal. 



§ 86. 



With the separation of the body into an ectodermal and an 

 endodermal layer, we get the lowest condition of the organs of 

 nutrition; the endoderm investing a space which is open to the 

 exterior, and which is the earliest distinct digestive cavity (stomach, 

 euteron) — (cf. supra, § 28). This condition is simplest in the Gastrula 

 form, and undergoes various differentiations in the two chief divisions 

 of the Coelenterata. The stomachal cavity, that is, does not remain a 

 mere simple space, but grows out into various kinds of cavities, 

 canals, pouches, and so on, which are either distributed irregularly 

 in the organism, or are arranged in a definite way. As a rule, 

 division of labour takes place at the same time, and only one 

 definite part, or several such parts, functions as a digestive cavity, 

 while the other spaces are used to distribute the nutrient fluid 

 (chyme). But this gastric system has other functions too. There 

 is no doubt that it also has a respiratory function, by distributing 

 through the body the water that was taken in with the food ; and 

 for this purpose it has, especially in the Spongite, a much more 

 extended surface than has the outside of the body. Finally, it has 

 important relations to reproduction, for the generative elements 

 are formed in its walls. 



§ 87. 



Among the SpongiaB the simplest form is limited to the early 

 stages of development, or is per- 

 manent as in the Gastrasades. 

 Almost the only complication 

 in the Gastrasades is the de- 

 velopment of an arrangement 

 for producing a current at the 

 entrance of the simple enteric 

 tube. In the Porifera there 

 are various new complications. 

 Temporary spaces appear in the 

 endodermal layer, which break 

 through to the exterior, so that 

 in addition to its mouth (Fig. 41, 

 o), the enteric cavity is con- 

 nected with the exterior by 

 pores at various points, which 

 open and again close. The number of these pore-canals (dermo 



Fig. 41. An Ascon-stock of nine persons 



(individuals). Diagram. e Ectoderm. 



i Endoderm. o Mouth, g Enteric cavity 



(after E. Hackel). 



