ECHINOIDS. 101 



only in assuming its adult form that the Peiitacta develops its 

 other ambulacra, with their many suckers. 



The Peiitacta resembles the Trepang, so highly valued by the 

 Chinese as an article of food, and forms a not unsavory dish, 

 having somewhat the flavor of lobster. 



ECHINOIDS. 



Sea-urchin. (Toxopneustes drobacldensis AG.) 



Sea-urchins (Fig. 131) are found in rocky pools, hidden away 

 usually in cracks and holes. They like to shelter themselves in 

 secluded nooks, and, not satisfied even with the privacy of such a 

 retreat, they cover themselves with sea-weed, drawing it down 

 with their tentacles, and packing it snugly above them, as if to 

 avoid observation. This habit makes them difficult to find, and 

 it is only by parting the sea-weed, and prying into the most 

 retired corners in such a pool, that one detects them. Their 

 motions are slow, and they are less active than either the Star- 

 fish or the Ophiuran, to both of v which they are so closely allied. 



Let us look at one first, as seen from above, with all its various 

 organs fully extended. (Fig. 131.) The surface of the animal is 

 divided by ten zones, like ribs on a melon, only that these zones 

 differ in size, five broad zones alternating with five narrower ones. 

 The broad zones, representing the interambulacral system, are 

 composed of large plates, supporting a number of hard projecting 

 spines, while the narrow zones, forming the ambulacral system, 

 are pierced with small holes, arranged in regular rows, (Fig. 132,) 

 through which extend the tentacles terminating with little cups 

 or suckers. Tliese zones converge towards the summit of the ani- 

 mal, meeting in the small area which here represents the dorsal 

 system ; this area is filled by ten plates, five larger ones at the 

 extremity of the interambulacral zones, and five smaller ones at 

 the extremity of the ambulacral zones. (Fig. 132.) In the five 

 larger plates are the ovarian openings, so called because each 



