96 



IXVERTEBKATE ANIMALS. 



cilia there are two very long and flexible tentacular processes, which 

 are fringed on one side by smaller secondary branches. The ten- 

 tacles arise each from a kind of sac, one placed on each side of the 

 body, and they can be instantaneously and completely retracted 

 within these sacs at the will of the animal. The mouth of Pleuro- 

 hrachia opens into a spindle-shaped digestive sac or stomach, which 

 in turn opens below into a wider and shorter cavity termed the 

 " funnel " ; from this there pi'oceed in the axis of the body two 

 small canals, which open at the opposite pole of the body. The 

 funnel conmiuuicates with a complicated system of canals, which 

 are ciliated internally, and are filled with a nutrient fluid. In the 

 angle between the two canals which run from the base of the 

 funnel to the surface is a little vesicle or sac, believed to be a 

 rudimentary organ of hearing, and placed upon this is a little mass 

 which is generally believed to be of a nervous nature. The repro- 

 ductive organs are developed in the walls of the canal-system. 



Fig. 59. — Ctenophora. Gestum Veneris, reduced in size. 



The only other form of the Ctenophora which deserves mention is 

 the "Venus's girdle" {Cestum Veneris), which agrees in essentials 

 with Pleurobrachia, but is greatly elongated in a direction at right 

 angles to the alimentary canal, till we have a ribbon-shaped body 

 produced (fig. 59), four or five feet in length and two or three inches 

 high. Cestum is not uncommon in the Mediterranean, and has the 

 powei' of phosphorescence, appearing at night as a movinc and twist- 

 ing band of flame. 



