TYPES OF CCELENTERA A MEDUSOID. 137 



the bell also bears tentacles, usually hollow, and abundantly 

 furnished with stinging cells (Fig. 55, 3). 



On the convex surface of the bell the ectoderm forms 

 simply an epithelial layer; on the concave surface it is 

 differentiated into muscle cells on the velum, the manu- 

 brium, and the tentacles, nerve cells at the base of the 

 velum, and stinging cells on the tentacles. The endoderm 

 is ciliated ; it lines the food space, and extends also into the 

 tentacles. The mesoglcea 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. 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 ganglionic cells, which apparently control the 

 muscular contractions. 



Sense organs may be present, in the form of " eyes," at 

 the base of the tentacles (Ocellatag), or in the form of 

 " auditory " vesicles developed as pits in the velum (Vesi- 

 culate). 



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 



