IV 



PHYLUM CCELENTERATA 



147 



Hermit-crabs is a case of commensalism : the hydroid feeds upon 

 minute fragments of the Hermit-crab's food, and is thus its com- 



FIG. 108. Hydra. A, vertical section of entire animal ; B, portion of transverse section, 

 highly magnified ; C, two large ectoderm cells ; D, endoderm cell of H. viridis ; E, large 

 nematocyst ; F, small nematocyst ; G, sperm, a, ingested diatom ; bd. 1, bd. 2, buds ; 

 chr. chromatophores ; cnbl. cnidoblast ; cnc. cnidocil ; ect. ectoderm ; end. endoderm ; 

 ent. cav. enteric cavity ; ent. cav'. its prolongation into the tentacles ; fl. flagellum ; hyp. 

 hypostome or manubrium ; int. c. interstitial cells : m. pr. muscle-processes ; mth. mouth ; 

 msgl. mesogloea ; ntc. large, and ntc'. small nematocysts ; nu. nucleus ; ov. ovum ; ovy. 

 ovary ; psd '. pseudopods ; spy. spermary ; vac. vacuole. (From Parker's Elementary 

 Biology, after Lankester and Howes.) 



mensal or messmate ; and the Hermit-crab is protected from its 

 enemies by the presence of the inedible, stinging hydroid. 



L 2 



