io6 Introduction to Animal Morphology. 



CHAPTER XVII. 



CTENOPHORA (Eschscholtz}. 



SUB-CLASS 5. CTENOPHORA.* Transparent, pelagic, 

 gelatinous forms, radially and bilaterally symmetri- 

 cal ; moving by meridional, comb-like rows of ciliated, 

 ectodermal swimming plates (ctenophores), between 

 which are interspaces or ctenomeres. There are 

 usually eight of these (four in Cestum, twelve in 

 Alcinoe) ; each plate is thick, and movable by muscle 

 at the base ; thinner, but rigid to the ciliated edge,, 

 where the central cilia are longer than the lateral. 

 Towards each pole of the often ovoidal or spheroidal 

 body the ctenophoral ridges end in rows of cilia, nar- 

 rowing finally to a point. The mouth is simple, or 

 with raised edges, at one end (oral pole), and opens 

 into a fusiform, tubular stomach, lined with coloured 

 hepatic (?) cells. From this passes a wide, short fun- 

 nel, to the apical pole, where it ends by two anal 

 openings (apical pores) ; or may be closed (Cestum?). 

 This pole is usually uppermost in swimming. From 

 the base of the funnel arise two paragastric canals,, 

 which pass one at each side of the stomach, towards 

 the oral pole ; and below these there are two or four 

 others (primary radial canals) passing horizontally ; 

 these (if two) bifurcate into secondary radial canals, 

 each of the four ending in a pair of tertiary canals; 

 one of these passes out to beneath each row of swim- 

 ming plates, and is there continued from pole to pole 



* This possibly should be a class, equivalent to Hydrozoa, and inter- 

 mediate between Discophora and Echinodermata ; to the latter of which 

 it has many affinities (Agassiz). 



