IV PHYLUM CCELENTERATA 117 



twisted cords, the mesenteric filaments {mes. F), answering 

 to the gastric filaments of Scypbozoa. Stinging- capsules 

 occur in the ectoderm, and are also very abundant in the 

 mesenteric filaments. They resemble in general character 

 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. 



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, etc. 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. 



Sea-anemones are dioecious, the sexes being lodged in 

 distinct individuals. The gonads — ovaries or testes — are 

 developed in the substance of the mesenteries (Fig. 54, 

 gori), a short distance from the edge, and, when mature, 

 often form very noticeable structures. The development 

 of sea-anemones resembles, in its main features, that of 

 Scyphozoa, but there is no alternation of generations. 



Our common sea-anemone, Metridium marginatum (Fig. 

 56), lives under stones near low-water mark. 



Two main divisions or sub-classes of the Actinozoa are 

 recognised, — the Zoantharia and the Alcyonaria, the 

 former including the sea-anemones, the Madrepores, and 

 other stony corals, and the horny black corals ; the latter 

 the "dead men's fingers," red coral, organ-pipe coral, 

 "sea-fans," and "sea-pens." The principal distinguishing 



