IV 



PHYLUM CCELENTERATA 



133 



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 andendoderm, 

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

 cells (see p. 32). 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 

 the latter retreats 

 from it, the con- 

 nection being 

 maintained only 

 at irregular inter- 

 vals. In the same 

 way the hydro- 

 and gono-thecse 

 are cuticular pro- 

 ducts of the 

 polypes and blas- 

 tostyles respec- 

 tively : in the 

 young condition 

 both occur in the 

 form of a closely 

 fitting invest- 

 ment of the 

 knob - like rudi- 

 ment of the zooid 

 (Fig. 96, Bd. 7, 2). 



The ectoderm 

 has the general 

 character of a 

 columnar epithe- 

 lium (see p. 26), 



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- 

 cells (Fig. 99, nv.c). The tentacles and the manubrium contain, 

 in addition, a layer of unstriped muscle-fibres between the ectoderm 

 and the mesoglcea : they are arranged longitudinally, and serve 

 for the rapid shortening of the tentacles (Fig. 99, mf). This 

 muscular layer is a derivative of the ectoderm, and may be looked 

 upon as a rudimentary mesoderm. 



Embedded in the ectoderm are numerous clear ovoid bodies, the 



FIG. 97. Obelia sp. Vertical section of a polype, highly 

 magnified ; ect. ectoderm ; wrf.Sendoderm ; ent. enteric cavity ; 

 h. 111. hydrotheca ; msgl. mesoglcea ; mth. mouth ; ntc. nemato- 

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



