THE ANTHOZOA 



and homogeneous, or whether it contain intrusive cells imbedded 

 in a homogeneous matrix. 



Intimately connected with the absence of a mesoblast is the 

 absence of all those cavities and structures which, in the higher 

 metazoa, are lined by or formed from the mesoblast. There are 

 no coelomic spaces in the Coelentera, no haemal or blood spaces, 

 no specialised respiratory or nephridial systems. The musculature 

 is derived either from the ectoderm or from endoderm, or in cases 

 in which mesogloeal muscles may be spoken of, their origin from 

 one or other of these layers is apparent. The same may be said 

 of the skeletal tissues. 



The Anthozoan zooid, whilst possessing the general features 

 enumerated above, differs from other Coelenterate zooids in some 

 important particulars. 



The mouth in such an animal as Hydra opens directly into 

 the coelenteron, and the external ectoderm passes into the 

 endoderm at its lips. In the Anthozoan zooid the mouth does 

 not open directly into the coelenteron, but into a shorter or 

 longer tube, which projects into the coelenteron and opens into it 

 below. This tube is formed in the course of development as an 

 invagination of the ectoderm, and is therefore a stomodaeum. It 

 is seldom round, more generally compressed from side to side, so 

 as to be oval or slit-like in transverse section. 



At either one or at both ends of the oval there is a groove, the 

 cells lining which are furnished with specially long cilia. When 

 only one groove is present, it is termed the sulcus (siphonoglyphe 

 of Hickson), where two grooves are present one is termed the 

 sulcus and the other the sulculus. The elongation of the mouth 

 and stomodaeum confers a bilateral symmetry on the Anthozoan 

 zooid, which is extended to other organs of the body. One may 

 speak of a sulcar and a sulcular aspect of the body in cases in 

 which two grooves are present, and of a sulcar and asulcar aspect 

 in cases in which only one groove is present. These terms are 

 preferable to the older terms "ventral" and "dorsal," which 

 cannot properly be applied to the Anthozoa, since they have 

 nothing corresponding to the ventrum and dorsum of higher 

 animals. It must be understood that, throughout this chapter, 

 the sulcar surface corresponds to the ventral surface of other 

 authors, the asulcar or sulcular surface to the dorsal. The terms 

 sulcus and sulculus and the corresponding adjectives are due to 

 Haddon (33). 



It is obvious from this description that the mouth of Hydra 

 and its allies does not correspond morphologically with what is 

 usually called the mouth, but rather with the inferior opening of 

 the stomodaeum of the Anthozoan zooid ; this being the region in 

 both groups at which the ectoderm passes into the endoderm. 



20 



