STRUCTURE OF MEDUSOID 



183 



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 contrac- 

 tions 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 associ- 

 ated ganglionic cells, which apparently control the muscular 

 contractions. 



Sense organs may be present, 

 in the form of " eyes," at the 

 base of the tentacles (Ocellatae), 

 or in the form of " auditory " 

 (probably balancing) vesicles 

 developed as pits in the velum 

 (Vesiculatae). 



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 repro- 

 ductive persons in 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 open- 

 ing, and usually without a central cavity. In the case of 

 those medusoids which arise as liberated sexual members of 

 a fixed asexual hydroid colony, the planula settles down, 

 loses its cilia, buds out tentacles, and develops into a new 

 hydroid. 



In many hydroid colonies, as has been already noticed, 



Fig. 92. — Structure of a Medu- 

 soid. — After Allman. 



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

 velum; T., tentacle; C.C., circum- 

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

 radial canal ; EN., endoderm ; 

 EC, ectoderm ; MG., mesoglcea. 



