354 ECHINODERMS OF THE BRITISH ISLES 



feebly developed. Very simple light-perceiving organs may occur ; 

 in Synaptids and Elpidiids there are small statocysts at the radial 

 canals. Small sensory cups, usually stalked bodies with a small 

 ciliated groove at the end, occur on the tentacles, sometimes also 

 on the body wall of Synaptids. The genital organs (gonads) 



are found only in the mid-dorsal 

 interradius ; they consist of a 

 number of branched or un- 

 branched long tubes, mostly on 

 each side of the mesentery, but 

 in some forms only on one side. 

 They all open through a single 

 duct imbedded in the mesentery, 

 on the dorsal side, mostly at the 

 base of the tentacles, sometimes 

 at the tip of one of the tentacles ; 

 in some deep - sea forms the 

 genital opening is situated farther 

 back on the dorsal side, some- 

 times even at the posterior end. 

 Not rarely the genital opening 

 lies at the tip of a small papilla. 

 The efferent duct is sometimes 

 divided, opening through several 

 pores {e.g. Cucumaria frondosa). 

 The Holothurians generally have 

 separate sexes, but several of 

 them, e.g. most Synaptids, are 

 hermaphrodite, usually so that 

 eggs and sperm develop in the 

 same gonad, but at different 

 times, the individual being thus 

 alternately male and female. 

 Some Holothurians are vivipar- 

 ous ; among the North European 

 forms this, however, is the case 

 with only one species, Lepto- 

 synapta minuta. Other, mainly 

 brood - protecting ; only a single 

 is known to protect its 



Fig. 212. — Anterior end of Holo- 

 thuria tuhulosa ; the body wall 

 of the left side removed in order 

 to show the inner anatomJ^ 

 (Modified, after Goodrich ; from 

 DanynarJc' s Fauna.) 



am, Tentacle ampullap, bent forwards in 

 order to show the calcareous ring ; bl r, 

 Circular blood-vessel; d bl, Dorsal blood- 

 vessel of intestine ; g, Efferent duct of 

 genital organs ; go, Genital organs (gonads) ; 

 h, The skin; kr, Calcareous ring; Im, 

 Longitudinal muscles ; ms, Mesentery ; 

 p, Polian vesicle ; po. Genital pore ; rk, 

 Radial canals ; rm, Circular muscles ; st, 

 Stone canals ; t, Alimentary canal ; te, 

 Tentacles ; v bl, Ventral blood-vessels ; vr, 

 Watervascular ring. 



Antarctic, Holothurians are 

 Arctic species, Cucumaria ] 

 brood. 



Several Holothurians, e.g. (probably) most of the Aspido- 

 chirotes and some Synaptids, have a pelagic larva, the so-called 



