190 



ZOOLOGY 



The general microscopic structure of a Sea-anemone is well 

 shown by a section through a tentacle (Fig. 139). Both ectoderm 

 ((•'■/.) and endoderm (end.) consist mainly of very long columnar, 

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

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

 being travereed by a network of delicate fibres with interspersed 

 cells. The middle la^-er has, in fact, ceased to be a mere gelatinous 

 supporting lamella or mesmjhx'it, and has assumed, to a far greater 



^^iJlF^^s^^iS^ — 'tnsg'l 



Fio. isn.— Tealia crassicomis. Trans- 

 vei'se section of teutacle. &rt. ectoderm ; 

 end. endodei-ni; l.yii. longitudinal muscles ; 

 'TifisgL, niesoglosa ; nr.r. nerve-cells; ac.f. 

 nerve -fibres ; ntc. nematocysts ; t. w. 

 transverse muscles. (After Hertwig.) 



FiC!. 140. — 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. 140) 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. 141, 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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