32 HYDRA. 



small, and not associated in distinct batteries, and 

 are all of one kind. Moreover, the cuticle, so 

 extremely delicate in Hydra, is thickened in Obelia 

 to form a protective coat, the perisarc, which ex- 

 pands as a chalice about each polyp. 



Obelia is easily found on the fronds of seaweed, and can 

 be kept alive in sea-water without any special precautions 

 other than frequent aeration and excluding dust. To fix 

 colonies use corrosive sublimate and acetic acid ; stain with 

 nigrosin and mount in balsam. 



In the axil or angle between the polyp and the 

 stem, shoots of a kind differing in many respects 

 from the ordinary polyps will be found. These are 

 the blastostyles, or genital sacs, and differ from 

 the hydroid polyps in having neither mouth nor 

 tentacles, and in giving rise to buds that acquire 

 mouth, tentacles, swimming-bell, and reproductive 

 organs. These buds are jelly-fish or medussB. 



B. Structure of a Medusa. 



An Obelia-medusa consists of a very shallow 

 bell, with the clapper hanging down from its centre. 

 The clapper is a greatly elongated throat or manu- 

 brium, opening at its lower end by the mouth, and 

 leading at its upper end into four radial canals that 

 run to the margin of the umbrella, and are there 

 connected by a circular canal. 



The bell consists of a gelatinous disc provided 

 with marginal circular muscles by which the pul- 

 sating movements of the medusa are performed. 

 Along the edge of the bell, tentacles are distributed 

 and eight otocysts are found distributed, two in 

 each quadrant at equal distances from one another. 

 These organs of balance probably subserve the 

 functions of rendering Obelia sensitive to water 

 movement. 



