CGELENTERATA. IO/ 



into the general cavity of the body (Actinozoa, fig. 35). In no 

 case is there a distinct intestinal canal. which runs through the 

 body and opens on the surface by a mouth at one end and an 

 excretory aperture or anus at the other. It should, however, 

 be mentioned here that some modern zoologists, such as Gegen- 

 baur and Haeckel, consider that the entire system of internal 

 cavities in any Ccelenterate is truly homologous with the intes- 

 tinal canal of other animals, and that the Ccelenterata, therefore, 

 possess ho true body-cavity at all. To this view some of our most 

 distinguished authorities, such as Professor Allman, have given 

 their adhesion ; and there is no doubt that there are weighty 

 grounds for regarding it as the correct explanation of the facts, 

 though any discussion of these grounds would be out of place 

 here. It need only be added that if this view be accepted, it 

 will entirely subvert the generally received conception of the 

 structure of the Ccelenterata as above expressed. 



Though of the true " radiate " type, some Coelenterates show 

 traces of bilateral symmetry. Thus, in some Sea-anemones one 

 of the tentacles is larger than, or differently coloured from, the 

 others ; and in some corals two of the primary septa, opposite 

 one another, are larger than the rest, and divide the animal into 

 two halves. 



With regard to the fundamental tissues of the Ccelenterata, 

 there exist two primary membranes, of which one forms the 

 outer surface of the body, and is called the " ectoderm ; " 

 whilst the other lines the alimentary canal, the general cavity 

 of the body, and the tuBuIaf "tentacles, and is termed the 

 " endoderm." These membranes correspond with the primi- 

 tive serous and mucous layers (" epiblast " and " hypoblast ") 

 of the germinal area, and become differentiated in opposite 

 directions, the ectoderm growing from within outwards, the 

 endoderm from without inwards. Each is primitively cellular 

 in its minute structure, and each may be rendered more or less 

 complex by vacuolation or fibrillation. Between the ectoderm 

 and endoderm there is sometimes a third layer (" mesoderm " 

 or " mesoblast "), which is commonly of a muscular nature. 



In connection with the integument of the Ccdenterata, the 

 organs termed "thread-cells" (" cnidae," or " nematocysts ") 

 must be noticed. These are peculiar cellular bodies (fig. 36), 

 of various shapes, which probably serve as weapons of offence 

 and defence, and which commimicate to many members of the 

 sub-kingdom (e.g., the Sea-Blubbers) their well-known power 

 of stinging. In the common Hydra the thread-cells (fig. 36, E) 

 consist of " oval elastic sacs, containing a long coiled filament, 

 barbed at its base, and serrated along its edges. When fully 



