TYPES OF CCZLENTERA A MEDUSOID. 



tentacles. The mesogloea forms a thickened jelly, present 

 more especially on the convex (ex-umbrellar) surface. 



The mouth opens into the canal of the manubrium, which 

 leads to the central cavity of the dome. With this a varying 

 number of unbranched radial canals communicate; these 

 open into a marginal circular vessel, which communicates 

 with the cavities of the tentacles. A plate of endoderm lies 

 in the mesoglcea between the radial canals. Digestion is 

 intracellular, and probably goes 

 on throughout the whole of this 

 " gastro- vascular " system. 



The movements of the bell 

 are caused by the contractions 

 of the ectodermic muscle cells. 



The nervous system consists 

 of a double ring of nerve fibres 

 around the margin of the bell. 

 With these are associated gang- 

 lionic cells, which apparently 

 control the muscular contrac- 

 tions. 



Sense organs may be present, 

 in the form of "eyes," at tr^e 

 base of the tentacles (Ocellatae), 

 or in the form of " auditory ;; 

 vesicles developed as pits in the 

 velum (Vesiculatse). 



The reproductive organs develop either in the manu- 

 brium or on the radial canals. The products always (?) 

 ripen in the ectoderm, and often seem to arise there ; but 

 Weismann and others have shown that the reproductive 

 cells of a medusoid derived from a hydroid, or of the 

 reduced and fixed reproductive persons of many hydroids, 

 have considerable powers of migration, and may originate 

 (sometimes in the endoderm) in the hydroid colony at 

 some distance from the place where they are matured within 

 the medusoid bud. The sexes are usually separate. The 

 commonest kind of free-swimming larva is the planula, which 

 is oval, ciliated, and diploblastic, devoid of an opening, and 

 usually without a central cavity. In the case of those 

 medusoids which arise as liberated sexual members of 



FIG. 72. Structure of a 

 Medusoid. After Allman. 



ST., Stomach ; M., manubrium ; V., 

 velum ; 7"., tentacle ; C.C., circum- 

 ference canal; G., gonad ; R.C., 

 radial canal ; EN. t endoderm ; 

 EC.t ectoderm ; MG., mesoglcea. 



