Chap. XVI 1 1.] 



THE HYDRA. 



343 



which is detached and floats off to enjoy a free and separate 

 existence as a provisional embryonic organism. 



Such a medusa is figured in Fig. 101. Within the bell-shaped 

 hollow umbrella or nectocalyx there hangs a tubular manubrium 

 (m.). The cavity of the manubrium leads up into the umbrella, 

 and there branches into four or more radiating canals (r. c.), 

 which proceed outwards and 

 lead into a circular canal (c. c. ), 

 running round the rim of the 

 nectocalyx. The rim has a deli- 

 cate shelf -like inward projection 

 the velum (v.) which narrows 

 the orifice of the bell. From 

 the edge of the rim tentacles (t.) 

 take their origin; and on its 

 margin there may be sense 

 organs, called marginal bodies 

 (m. b.), which may be eyespots 

 or auditory vesicles. The sper- 

 maries and ovaries are generally 

 in different individuals, and are 

 lodged either in the walls of the 

 manubrium, or in those of the 

 radiating canals (g. o.). 



The nervous system consists 

 of a double ring of nervous 



matter, containing both nerve-fibres and nerve-cells, which runs 

 round the edge of the nectocalyx at the point of insertion of the 

 velum. The upper nerve-ring lies entirely in the ectoderm, and 

 is hardly differentiated from the epithelium. It sends out fibres 

 to the enveloping epithelium, and these, or the epithelium cells, 

 bear delicate sense-hairs. The lower nerve-ring, though still 

 belonging to the ectoderm, is more completely differentiated 

 from the epithelium, has larger fibres, and a greater number of 

 ganglion cells. The two nerve-rings are separated by a thin 

 membrane, through which, however, there pass strands of con- 

 necting fibres. The entire under surface of the nectocalyx 



FIG. 101. MEDUSA. 



c. c. Circular canal, g. o. Genital organ. 

 m. Manubrium. m. 6. Marginal body 

 (ocellus or otocyst). r. c. Radiating canal. 

 t. Tentacles, v. Velum. 



