CCELENTERATA : ACTINOZOA. 131 



(Greene.) The normal number of the ctenophores appears to 

 be eight, and each consists of a band of surface elevated trans- 

 versely 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, Pleurobrachia 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 instan- 

 taneously retracted at the will of the animal. 



Fig. 36. — Ctenophora. Plenrobrachia pileus. 



The mouth of Pleurobrachia (fig. 37, a) opens into a fusi- 

 form digestive sac, or stomach (b), the lower part of which is 

 provided with brown cells, supposed to discharge the functions 

 of a liver. The stomach opens below into a shorter and wider 

 cavity {c), termed the " funnel," from which two canals diverge 

 in the direction of the vertical axis of the organism, to open 

 at the " apical pole." These canals are known as the " apical 

 canals" (e), and their apertures as the "apical pores." From 

 the funnel two other pairs of canals are given off. Of these, 

 one pair — known as the " paragastric canals " — turns upwards, 

 one running parallel to the digestive sac on each side (</), and 

 " terminating caecally before quite reaching the oral extremity." 

 The second pair of canals {i) — the so-called "radial canals" — 

 branch off from the funnel laterally, each dividing into two, 



