252 



C(ELENTERATA. 



relatively long tongue-like processes, and grow out from the disc-like 

 segments of the titrobila as marginal cones. An undivided mar- 

 ginal membrane (the velarium], differing from the velum of the 

 Craspedota [in containing prolongations of the canals of the gastro- 

 vascular system], is present in the Ckarybdeidce alone. 



The Acalepha differ from the Hydro niedusce in possessing, as a rule, 

 large oral tentacles at the free end of the wide manubrium. These 

 may be regarded as being derived from an unequal growth of the 

 edges of the mouth. They grow as four arm-like processes of the 

 manubrium from the angles of the mouth, and are placed radially, 



FIG. 195. Diagrammatic longitudinal section through a Bhizosfonm. U, Umbrella; M, 

 gastric cavity; S, sub-umbrella; &, genital band; Sh, sub-genital pit; F, filament; 

 SM, muscle system of the sub-umbrella ; R.gf, radial vessels ; Rk, sense organs ; 

 Rg, olfactory pits ; Al, ocular lobe ; 8k, shoulder tufts ; Dk, dorsal tufts ; f'k, ventral 

 tufts of the eight arms ; Z, terminal parts of the arms. 



i.e. they alternate with the genital organs and gastric filaments. 

 In some cases the arms become forked at an early period, and four 

 pairs of arms are formed, the lobed tufted edges of which may again 

 divide and sub-divide into many branches. In this case, the margins 

 of the mouth and the opposed surfaces of each pair of arms fuse in 

 early life in such a way that the original central mouth becomes 

 obliterated, and in its place there are developed a number of small 

 tufted orifices on' the peripheral parts of the arms, through which 

 nutriment is taken in (Rhizostomidse, fig. 195). 



