32 PHYSIOLOGY OF THE INVERTEBRATA. 



thrown into the gastro-vascular cavity, and collectively ex- 

 pelled through the exhalent aperture or apertures (Fig. 4, a), 

 as the case may be. 



The Porifera are composed of three layers ; the ectoderm 

 (of flat epithelial cells), the mesoderm, and the endoderm 

 (of long flagellate cells, Fig. 4, b). Besides a flagellum, a 

 single endodermic cell contains one or more contractile 

 vacuoles and a nucleus. It is possible that these endo- 

 dermic cells have the power of digesting the food particles, 

 and thereby rendering the food into such a state that it is 

 readily absorbed. The gastro-vascular cavity, with its in- 

 ternal lining of endodermic cells, is a rudimentary form of 

 ■digestive system. 



The Ccelenterata.* 



This class of the Mekizoa is divided into two sub-classes — 

 the Hijdrozoa and the Actinozoa. They have a mouth, a 

 gastro-vascular cavity, but no inhalent or exhalent apertures ; 

 in this point they differ from the Porifera. 



The morphological characteristic of the Ccdcnteratct is a body 

 with a constant cavity, which may be considered a digestive 

 cavity, but sometimes it is badly differentiated. 



In the case of the Hydra, it was formerly stated that if 

 the animal were turned inside out, it can digest with what was 

 previously its external surface : that is, the ectoderm and 

 endoderm were interchangeable. But recent Japanese ex- 

 periments have shown that this organism is more specialised 

 than was originally supposed to be the case ; and it has been 

 shown that when turned inside out, the Hydra again turns 

 itself back to its normal condition, so that the functions 

 of its inner and outer surfaces are not interchangeable. 

 Although in the Hydra the digestive cavity is somewhat badly 

 differentiated, in many of the Hydrozoa the digestive system 

 is divided into three parts — viz., an oesophageal portion, a 



* /co?\os, ivTtpov. 



