8;6 



SPONGES AND ZOOPHYTES 



stomach, which occupies M considerable proportion of the disc, and 

 projects downwards in the form of a proboscis, in the centre of which 

 is the quadrangular mouth (fig. 665, A, B). As the animal advances 

 towards maturity the intervals between the segments of the border 

 of the disc gradually fill up, so that the divisions are obliterated ; 

 tubular prolongations of the stomach extend themselves over the 

 disc ; and from its borders there sprout forth tendril-like filaments 

 which hang down like a fringe around its margin. From the four 

 angles of the mouth, which, even in the youngest detached animal, 

 admits of being greatly extended and protruded, prolongations are 

 put forth, which form the four large tentacles of the adult. The 

 young Medusas are very voracious, and grow rapidly, so as to attain 



FIG. 665. Development of Chrysaora from Hydra tuba : A, 

 detached individual viewed sideways, and enlarged, showing 

 the proboscis a, and b the bifid lobes ; B, individual seen from 

 above, showing the bifid lobes of the margin, and the quadri- 

 lateral mouth ; C, one of the bifid lobes still more enlarged, 

 showing the rudimentary eye (?) at the bottom of the cleft ; 

 D, group of young Medusae, as seen swimming in the water, 

 of the natural size. 



a very large size. The Cyanece and Chrysaorce, which are common all 

 round our coasts, often have a diameter of from six to fifteen inches ; 

 while Rhizostoma sometimes reaches a diameter of from two to three 

 feet. The quantity of solid matter, however, which their fabrics con- 

 tain is extremely small. It is not until adult age has been attained 

 that the generative organs make their appearance, in four chambers 

 disposed around the stomach, which are occupied by plaited mem- 

 branous ribbons containing sperm-cells in the male and ova in the 

 female j and the embryos evolved from the latter, when they have 

 been fertilised by the agency of the former, repeat the extraordinary 

 cycle of phenomena which has been now described, developing them- 

 selves in the first instance into hydroid polypes, from which medusoids 

 are subsequently budded off. 



