94 INVERTEBRATE ANIMALS. 



divided into a number of crescentic lobes by eight ciliated 

 bands or ctenophores, which proceed from near the mouth to 

 near the opposite pole of the body. Besides the 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. Ihe 

 mouth of Pleurobrachia 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 proceed 

 in the axis of the body two small canals, which open at the 

 opposite pole of the body. The funnel communicates 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 sur- 

 face 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. If this is 

 correct, this is the first indication which we have hitherto en- 

 countered of a genuine nervous system. The reproductive 

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



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 enlon- 

 gated in a direction at right angles to the alimentary canal, 

 till we have a ribbon-shaped body produced, four or five feet 

 in length and two or three inches high. Cestum is not un- 

 common in the Mediterranean, and has the power of phospho- 

 rescence, appearing at night as a moving and twisting band 

 of flame. 



