252 



GENERAL BIOLOGY 



grasping the hydroid form by the mouth and pushing this portion of the 

 animal in upon itself, when we have the gastrula still, and also the 

 medusoid form. 



It must be remembered that some species may always retain the 

 medusoid form and others the hydroid, while still a third may alternate 

 regularly or irregularly between the two. 



Obelia ( ) 



This is a colonial form of Hydra found attached to rocks, wharves, 

 and to various algae, in which budding began, but the newly-formed ani- 

 mals remained attached to the parent stalks. (Fig. 156.) 



Such a colony consists of a basal stem, the hydrorhiza ( ), 



which is attached to the substratum. At intervals, upright branches 

 known as hydrocauli ( ) are given off. At every 



bend in the hydrocaulus a branch arises. The stem of this side-branch 



Fig. 156. Hydrozoa. 



A, part of a colonial Hydrozoan, Obelia. B, Longitudinal sec- 

 tion through a single hydranth. C, Cross section through medusoid 

 individual. 1, ectoderm ; 2, entoderm ; 3, mouth ; 4, coelenteron ; 5, 

 coenosarc ; 6, perisarc ; 7, hydrotheca ; 8, blastostyle ; 9, medusa- 

 bud ; 10, gonotheca ; or.c, mouth region ; end. and endt., entoderm ; 

 ect., ectoderm ; st.l., mesoglea lying between ectoderm and ento- 

 derm ; hyth, hydrotheca. (From Borradaile after various authors). 



is ringed and expanded at the end into a hydra-like structure, the 

 hydranth ( ). Each individual polyp consists of a 



hydranth and the part of the stalk between the hydranth and the point 

 of origin of the preceding branch. Full-grown colonies usually bear re- 

 productive members (gonangia) in the angles where the hydranths arise 

 from the hydrocaulus. 



All of the soft parts of the Obelia colony are protected by a chitinous 

 covering called the perisarc ( ) which is ringed at 



