CCELENTEKATA I ACTINOZOA. 



Ill 



PleurobracMa (Cydippe) (fig. 29) may be taken as the type 

 of the order, the structure of all being similar to this in essen- 

 tial points. Pleurobrachia possesses a transparent, colourless, 



Fig. 29. Ctenophora. Pleurobrachia pileus. 



gelatinous, melon-shaped body, or * actinosoma,' in which the 

 two poles of the sphere are termed respectively the ' oral' and 

 * apical,' and the rest of the body constitutes the ' interpolar 

 region.' At the oral pole is the transverse mouth, bounded 

 by lateral, slightly protuberant margins. ' Eight meridional 

 bands, or " ctenophores " bearing the comb-like fringes, or 

 characteristic organs of locomotion, traverse at definite 

 intervals the interpolar region, which they divide into an 

 equal number of lune-like lobes, termed the " actinomeres ; " 

 but this division of the body does not extend into the imme- 

 diate vicinity of the poles, before reaching which the cteno- 

 phores gradually diminish in diameter, each terminating in a 

 point.' (Greene.) The normal number of the ctenophores 

 appears to be eight, and each consists of a band of surface 

 elevated transversely into a number of ridges, to each of 

 which a fringe of cilia is attached, so as to form a comb-like 

 plate. The cilia in the middle of these transverse ridges are 

 the longest, and they gradually diminish in length towards 

 the sides, so that the form of each comb is somewhat crescentic. 

 Besides the comb-like groups of vibratile cilia, Pleurobracliia is 

 provided with two very long and flexible tentacular processes, 

 which are fringed on one side with smaller cirrhi. These 

 filamentous processes arise each from a sac, situated on one of 

 the lateral actinomeres, within which they can be completely 

 and instantaneously retracted at the will of the animal. 



