SEA-CUCUMBERS. 



39 



madreporic plate, w h, and a similar circulatory system are 

 found as in the star-fishes. 



Development. They are produced from eggs, the free- 

 swimming young passing through many changes (Fig. 37). 

 In some Echini the young are pro- 

 duced without changes. The Her- 

 miaster holds its young in a regular 

 sac, while the eggs and young in 

 others are held in place by the 

 spines that fold about them like so 

 many protective arms. 



Class IV. SEA -CUCUMBERS 

 (Holothuroided). 



In these animals (Fig. 38) the 

 body is long and worm-like, and 

 resembles a rough-skinned cucum- 

 ber. Looking down upon the 

 mouth, that is surrounded by ten- 

 tacle-like gills, their radiate char- 

 acter is seen, and by laying open 

 the body a similar disposition of 

 muscles may be noticed. The 

 madreporic plate or strainer is in- 

 ternal. The feet are in five se- 

 ries, each consisting of five rows, 

 by which they move slowly along. 



The skin of Holothurians is leathery, and contains num- 

 bers of curious bodies resembling dumb-bells, wheels, 

 and anchors (Fig. 39). The Holothuria Floridiana is in- 

 habited by a fish, the Ficrasfer* In the Holothuria sea- 



* In many observed by the author on the Florida reef they invariably 

 died as soon as taken from the Holothurians ; but in the aquarium at 

 Naples they have been seen to leave their home and return tail first, 

 the action of the Holothurian in taking in water helping them in. 



FIG. 37. Development of 

 sea-urchin. A, free- 

 swimming young ; a, a, 

 lime rods ; 6, urchin 

 forming within. B, later 

 form, showing spines. 



