Coelentera. 101 



some portions being mucli richer in muscle cells, ganglion cells, 

 etc., than others. It may also be mentioned here that, in some 

 Coelentera, part of the body is surrounded by a cuticle, secreted 

 by the ectoderm ; further, certain portions of the ectoderm may be 

 modified to form simple optic or auditory organs. 



The endoderm, corresponding with the epithelium lining the 

 alimentary canal of other Metazoa, and originating from the 

 endoderm of the gastrula, is, structurally, very like the ectoderm, 

 since besides simple epithelial cells, epithelio-muscle cells, muscle 

 cells, sensory cells, ganglion cells, thread cells, and gland cells may 

 also be present. As regards details, e.g., the form of the cells, the 

 ■differences between the two layers are by no .means insignificant.* 



The mesogleea in some forms is a thin structureless sheath 

 without cells. In others it is better developed, and cells from the 

 ectoderm and endoderm migrate into it, so that it becomes like a 

 connective tissue. Occasionally, in the Ctenophora, some of the 

 cells which have wandered in, develop into muscle and ganglion 

 cells. In the mesoglsea of some Ocelentera hard structures may 

 develop, but these will be described later. 



Ova and spermatozoa develop in the ectoderm in some 

 cases, in the endoderm in others, arising as the modification of 

 the ordinary cells of the layer. In general they are developed 

 in definite regions of the body, which may then be termed, by_ 

 analogy, ovaries and testes, but in some forms they are irregularly 

 distributed. 



Prom the foregoing description it will be seen that there is 

 extraordinary simplicity of structure among the Coelentera. For 

 the most part the animal consists throughout life of the two layers of 

 the gastrula, the only further development being the various 

 modifications of the constituent cells. An actual mesoblast, arising 

 in other Metazoa as a special mass or masses of cells from which 

 large portions of the body are developed, is absent. The muscle 

 elements and the sexual cells (ova and spermatozoa), which are 

 elsewhere formed from the mesoblast, develop here from the ectoderm 

 and endoderm. For the most part it is almost impossible to speak of 

 organs as in other Metazoa; in any case there is very little 

 specialisation. Thus, a central nervous system which, even in very 

 simple Metazoa, is usually well-developed, can hardly be distinguished; 

 at most, the ganglion cells are more closely collected in some places 

 than in others. Excretory, vascular, and respiratory organs there 

 are none ; and a body-cavity is never developed. 



* In the inner layer of various Coelentera (Actinia, Mediisae, Hydroids), great 

 numbers of green or yellow cells are frequently found, each siu-rounded by 

 a definite cellulose cell-wall. These were formerly regarded as a part of the 

 animal, but as a matter of fact they are unicellular plants (Algae), which have 

 taken up their abode within the organism (c/. Eadiolaria). 



