100 



COELENTERATA. 



R. Leuckart was the first to recognise the importance of these 

 characters, and made use of them to separate the Polyps and the 

 Medusae from the Ecliinoderms, thus resolving Cuvier's type of 

 Radiata into the types of Coelenterata and Ecliinodermata. 



The Coelenterata are divided into two main sub-phyla, the Cnidaria 

 and the Ctenopliora, distinguished from one another by a number of 



characters, of which, perhaps, the most compre- 

 hensive is the presence or absence of nemato- 

 cysts. The entire structure of the body is 

 generally speaking disposed in radial symmetry, 

 although amongst the Cnidaria transitions 

 towards bilateral symmetry are sometimes 

 apparent. 



Three distinct types of body-form are met 

 with amongst the Coelenterata, viz., that of the 

 Polyp ; of the Medusa; and of the Ctenoplwre. 

 The Polyp type. The Polyp has the form 

 of a cylinder or sac (Fig. 79), of which, the 

 posterior or lower end is fixed and the opposite 

 end is free and pierced by an opening the 

 mouth placed on a flat or conical prominence 

 the oral cone or hypostome, and leading into 

 the cavity of the body, or enteric space (coelen- 

 teron). Around the mouth are placed a number 

 of regularly or irregularly arranged contractile 

 processes the tentacles, which always contain endoderm, either solid 

 or traversed by a prolongation of the enteric space. The tentacles 

 may be reduced to knob -like warts or be absent altogether 



(siphonozooids of 

 the Stylasteridae, 

 etc.). In rare cases 

 (Arachnactis, Min- 

 yas) the polyp is 



free-swimming. 



The ectoderm, 

 which is the part 

 of the polyp in 

 closest relation 

 with the outer world, possesses, to use the ordinary parlance of 

 histology, sense-cells (Fig. 80) provided Avith sensory hairs ; nerve or 

 ganglion-cells with branching processes ; epitlieUo-mmde cells, with 



FIG. 79. Diagrammatic 

 longitudinal section of 

 a liydroicl polyp. 

 mouth ; M enteric 

 space or coelenteron ; 

 Ek ectoderm ; En en- 

 dodenn ; T tentacle. 



&J&M2)e. E 



FIG. 80. Longitudinal section through the nerve ring of Charyb- 

 dea. Sz sense cells in the ectoderm ; Gz ganglion cells ; Nf 

 nerve-fibres ; Stl supporting lamella ; E endoderm cells. 



