IV 



PHYLUM CCELENTERATA 



189 



ntc 



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

 extremely 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 mesoglcea, and has assumed, to a far greater 

 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. 141) 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 undis- 

 charged capsule (A). 

 Gland - ceUs (Fig. 

 142, gl.) are very 

 abundant in the 

 ectodermal lining 

 of the gullet and in 

 the mesenteric fila- 

 ments : the latter 

 are trilobed in sec- 

 tion, and the gland- 

 cells are confined to 

 the middle portion, 

 the lateral divisions 

 being invested with 

 ordinary ciliated 

 cells (c.). In virtue 

 of possessing both 

 stinging - capsules 

 and gland-cells, the 

 mesenteric filaments perform a double function. The animal is very 

 voracious, and is able to capture and swallow small Fishes, Molluscs, 

 Sea-urchins, &c. The prey is partly paralysed, before ingestion, by 

 the nematocysts of the tentacles, but the process is completed, after 

 swallowing, by those of the mesenteric filaments. Then as the 

 captured animal lies in the stomach, the edges of the filaments 

 come into close contact with one another and practically surround 

 it, pouring out, at the same time, a digestive juice secreted by their 

 gland-cells. 



The muscles described above consist partly of spindle-shaped 

 nucleated fibres, and partly of muscle-processes, like those of 



FIG. 142. Transverse section of rnesenteric filament of 

 Sagartia. c. ciliated cells ; gl. gland-cells ; ntc. nematocysts. 

 (After Hertwig.) 



