CTENOPHORES AND JELLY-FISH 391 



most widely distributed forms belong to the group typified by the elegant melon 

 jelly-fish (Beroe ornata), so called from the oval shape of the delicate translucent 

 body, which is greyish white or pinkish in colour, and may measure as much as 

 six inches across. This beautiful organism is as common near the shore as in the 

 open sea ; but the allied B. forslcalia of the Atlantic is a much rarer type, dis- 

 tinguished by the somewhat flattened body. When young, this species is quite 

 transparent, but subsequently becomes rose-coloured, with a brown tinge on the 

 narrow sides of the body. One of the most beautiful of all free-swimming oceanic 

 forms is the so-called Venus' girdle (Cestus veneris), which is iridescent in 

 sunshine and phosphorescent at night, and has the body expanded into the shape 

 of a ribband, with the mouth placed midway on the lower edge. 



With regard to the ordinary branching and graceful animals constituting 

 the section Hydroida, it must suffice to state that these form horny, plant-like 



growths, in which the termination of each branch carries a polyp. All these polyps 

 are organically connected through the medium of tissues in the interior of the 

 branches ; but they are of two totally distinct types, one of which is essentially a 

 little elongated bag or tube, crowned with a ring of tentacles, and thus strictly 

 comparable to the fresh-water hydra, while the other, which in many cases 

 ultimately becomes detached and swims away to enter on a pelagic existence, is 

 practically a jelly-fish or medusa. These medusas are bell-shaped, translucent 

 organisms, with the mouth and digestive organs hanging from the centre of the 

 concavity of the bell, or, as it is often called, umbrella. In the walls of this 

 pendent mass are developed the eggs. Locomotion is effected by alternate con- 

 tractions and expansions of the umbrella. A colony of these hydroids, such as that 

 of Bougainvillea fracticosa, accordingly comprises one set of individuals the 

 function of which is to procure food, and a second set the duty of which is to 

 develop and carry eggs. The hydroid colony may directly develop eggs without 

 giving rise to free-swimming medusas, but when the latter are produced, these 

 fertilised eggs give rise to new stationary colonies, so that we have here a perfect. 



