IV 



PHYLUM CtELENTERATA 



177 



The general microscopic structure of a Sea-anemone is well- 

 shown by a section through a tentacle (Fig. 126). Both ectoderm 

 (cr/.) and endoderm (end.) consist mainly of very long columnar, 

 ciliated, epithelial cells, and the mesogloea (nisgl.) is not only ex- 

 tremely thick, but has the general characters of connective tissue, 

 being traversed by a network of delicate fibres with interspersed 

 cells. The middle layer has, in fact, ceased to be a mere gelatinous 

 supporting lamella or mesogloea, and has assumed, to a far greater 



ntc 



Fin. 12G. Tealia crassicornis. Trans- 

 verse section of tentacle, cet. ectoderm ; 

 end. endoderm; l.m. longitudinal muscles ; 

 msgl. mesoglcea ; ni:e. nerve cells; nv.f. 

 nerve fibres; utr. iiematocysts ; t. m. 

 transverse muscles. (After Hertwig.) 



FIG. 127. Three nematocysts of 

 Sagartia. (After Hertwig.) 



extent than in any of the lower groups, the characters of an inter- 

 mediate cell-layer or mesoderm. 



Stinging-capsules occur in the ectoderm, and are also very 

 abundant in the mesenteric filaments. They (Fig. 127) resemble 

 in general characters the nematocysts of Hydrozoa, but are of 

 a more elongated form, and the thread is usually provided at 

 the base with very numerous slender barbs (B). Very fre- 

 quently the coiled thread is readily seen in the undischarged 

 capsule (A). Gland-cells (Fig. 128, gl) are very abundant 

 in the ectodermal lining of the gullet and in the mesenteric 

 filaments : the latter are trilobed in section, and the gland- 

 cells are confined to the middle portion, the lateral divisions 



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