146 



ZOOLOGY 



.SECT. 



of red or black pigment at the bases of the tentacles. These ocelli 

 (oc) consist of groups of ectoderm cells containing pigment, and it 

 has been proved experimentally that they are sensitive to light : 

 they are, in fact, the simplest form of eyes. In the Leptomedusse 

 the umbrella is usually less convex, thinner, and of softer consis- 

 tency than in the Anthomedusae, the gouads are developed as buds 

 formed in connection with the radial canals and projecting from 

 the sub-umbrella, the velum is feebly developed, and sense-organs 

 take the form sometimes of ocelli, but usually of lithocysts. 



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FIG. 107. Ceratella fusca. About uat. size. (From Hickson, after Baldwin Spencer.) 



In the majority of Leptolinae the ccenosarc, as in Obelia, con- 

 sists of a more or less branched structure attached to stones, timber, 

 seaweeds, shells, &c., by a definite root-like portion (hydrorkiza). The 

 curious genus Hydractinia (Fig. 106, 7) is remarkable for possessing 

 a massive coenosarc, consisting of a complex arrangement of 

 branches which have undergone fusion, so as to form a firm 

 brownish crust on the surfaces of dead gastropod shells inhabited 

 by Hermit-crabs. The constant association of Hydractinia with 



