132 



ZOOLOGY 



SECT. 



coenosarc, blastostyles, and medusa-buds all consist of the same 

 layers, which are thus continuous through the entire colony. 



The perisarc or transparent outer layer of the stem shows no 

 cell -structure, but only a delicate lamination. It is, in fact, not a 

 cellular membrane or epithelium, like the ectoderm and endoderm, 

 but a cuticle, formed, layer by layer, as a secretion from the ectoderm 

 cells (see p. 31). It is composed of a substance of chitinoid or horn- 

 like consistency, and, like the lorica of many Protozoa, serves as a 

 protective external skeleton. When first formed it is of course in 

 contact with the ectoderm, but when the full thickness is attained 



FIG. 96. Obelia sp. Vertical section of a polype, highly magnified ; cct. ectoderm ; end. endo- 

 derm ; ent. enteric cavity ; h.th. hydrotheca ; msgl. mesoglcea ; mth. mouth ; ntc. nematocysts ; 

 sh. shelf -like prolongation of hydrotheca ; t. tentacle. 



the latter retreats from it, the connection being maintained only 

 at irregular intervals. In the same way the hydro- and gonothecse 

 are cuticular products of the polypes and blastostyles respectively : 

 in the young condition both occur in the form of a closely fitting 

 investment of the knob-like rudiment of the zooid (Fig. 95, Bd 1, 2). 

 The ectoderm has the general character of a columnar epithelium 

 (see p. 24), but exhibits considerable differentiation of its component 

 cells. It is mainly composed of large conical cells with their bases 

 outwards, and having between their narrow inner ends clumps of 

 small rounded interstitial cells, and occasional large branched nerve- 



