HEDGEHOG-SKINNED ANIMALS 



357 



large central one. These apertures no doubt enable a free cir- 

 culation of sea-water through the star-fish nursery to take place, 

 a matter of considerable import- 

 ance, since developing embryos 

 require a plentiful supply of oxy- 

 gen for breathing purposes. A 

 simpler case is that of a Star- 

 Fish [Leptopty chaster, fig. 877) in 

 which the young are developed 

 in grooves on the upper surface 

 of their mother, on whose back 

 they creep about after hatching 

 out. 



In most Sea-Ciicumbers i^Holo- 

 tluiroided) the eggs hatch out into 

 a simple kind of free - swimming 

 larva (Auricularia, fig. 878), but 

 in a few species there is direct 

 development. In such cases the 

 young are either simply attached 



to the skin of the parent without any special protective arrange- 

 ment, or the upper side 

 of the body may bear 

 (as in Psolus ephippifer) 

 a number of plate -like 

 spines with narrow stalks. 



Fig. 876.- 



-Heart-Urchin {Amphidotus cordntus) 

 in its Burrow 



As indicated by the arrows, food is entering by pas- 

 sage on left (through which some tube-feet, c, are 

 protruding) to mouth [a], and waste matters {d) are 

 being discharged through passage on right from intes- 

 tinal aperture [b): e, rosette of respiratory tube-feet. 



Fig. 877. — Young Star-fishes [Leptoj>tychaster Kergue- 

 lettsis) creeping on the Body of their Mother 



-M 



-I A 



Fig. 878. — Auricularia, en- 

 larged : M, mouth ; 1 a, intes- 

 tinal aperture. 



The broad tops of these overlap, to roof in a space that serves 

 as a brood-pouch. 



