126 



Echinoderma. 



in tlie epidermis, wliilst in other forms they have sunk farther in. As 

 to sense organs, the small eyes present at the tips of the 

 arms of Starfish must be mentioned. Small eyes are also present in 

 certain Holothurians {Synapta), at the base of the tentacles; and 

 lastly, optic organs have been described in some of the Sea-urchins 

 where they are distributed in larger numbers over the whole surface 

 of the body. Yesicular auditory organs are known only in 

 some of the Holothurians. 



Reproduction, with a few exceptions, is sexual, and the 

 Echinoderms are almost all of separate sexes. The male and 

 female organs are usually very similar in form, but they may generally 

 be distinguished without microscopic examination, since the ovary is 

 yellow or red, the testis white. As a rule they are radially arranged, 

 in each inter-radius an ovary or testis, or a small group of these; some- 

 times they are absent from one or more inter-radii, as in Irregular 

 Sea-urchins ; or they may be present in one only, as in the Holothurians. 

 They may be saccular or branched, and each opens by a pore upon its 

 inter-radius; in some forms near to the aboral pole, in others at some 

 distance from this ; or again, quite close to the mouth. 



Fertilisation generally occiu-s after the deposition of the ova, whicli 

 are usually small. Some few Echinoderms ai-e, however, viviparous, and 

 in these, of coui'se, fertilisation takes place within the body of the parent. In 

 some forms the ova ai-e can-ied about by the parent, either protected by the 

 spines or in special pits in the siu-face of the body ; some Starfish foita a kind 

 of brood-poTich by bending the arms down over the eggs. 



The development of the Echinoderms is of special interest, 

 for a complicated metamorphosis often occurs : the larval 



rig. 80. Diagrammatic figures of the principal forms of young Echinoderm larvte. 

 A, B, C seen from below ; A' i& A from the left side, a anus, / ciliated ridge, m month. 

 The saddle-shaped concave region is shaded. For the rest, see the text. — Orig. 



form, unlike the adult, shows no trace of a radial structure, but on 

 the contrary is bilaterally symmetrical, and all groups 

 conform to a common type, excepting the Crinoids and a few 

 others. The simplest form (see Fig. 80), seen in young larvae, 



