SEA-CUCUMBERS. 39 
madreporic plate, w, and a similar circulatory system are 
found as in the star-fishes. 
Development.—TVhey 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- 
master 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 
(Holothuroidea). 
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. 
FIG. 37.—Development of 
sea-urchin. A, free- 
swimming young ; 4, 2, 
lime rods; 4, urchin 
forming within. B, later 
form, showing spines. 
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 Merasfer.* In the Holothuria sca- 
* 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. 
