Echinoderms. 65 



contains, and then throw it out again. The Starfishes have no teeth, 

 but the walls of their alimentary canal secrete such strong juices that 

 they can kill animals which they have caught with their sucker-feet and 

 brought to their mouth. Their food consists chiefly of shellfish, but 

 often they will capture fishes and crabs. They are one of the most 

 formidable enemies of fishery, especially of oyster-culture. 



Like some of the Sea-urchins, most Sea-cucumbers live by swal- 

 lowing sand and mud and digesting the adhering organic particles. Some 

 (e. g. Cucumaria, Fig. 161) manage matters very differently. They remain 

 motionless on a stone 

 or other prominent ob- 

 ject, stretching out their 

 large branching tentac- 

 les and carrying them 

 one after the other into 

 their mouth, sucking off 

 the small animals which 

 in the meantime have 

 settled upon them. With 



a little patience this Fig . 1 6L Cacuman Planci, ' , nat. size, 



proceeding may be easily 



observed. - Some Sea-urchins (e. g. Dorocidaris, Fig. 8) have very 

 long spines; in these cases, however, the sucker-feet can be extended to 

 a great length, for they must always reach beyond the spines. In many 

 cases we find between the spines special moveable stalks bearing small 

 pincers, which can also seize small objects. The Starfishes have their 

 eyes at the extremities of their arms. They can, however, probably not 

 see very distinctly, but only distinguish between light and darkness. 



The Echinoderms are divided into four large groups : 



1. The Feather-stars .Crinoidea), 2. the Sea-stars (Asteroidea) 3. the 

 Sea-urchins (Echinoidea), 4. the Sea-cucumbers (Holothurioidea). 



Of the Feather -stars the Aquarium contains Antedon rosacea 

 (Fig. 5) in straw-coloured, orange-coloured, blood-red, or spotted brown 

 and white varieties. Generally they hold on to coral-branches, so as to 

 appear like flowers, growing on submarine trees. 



The Sea-stars are divided into the Starfishes or Asterids, and the 

 Brittle-stars or Ophiurids. The Starfishes are represented in the Aquarium 

 by the genera Luidia (Fig. 6), Astropecten (Fig. I), Asterias (Fig. 3), 

 Echinaster Fig. 2), Palmipes, and others; the Brittle-stars by Ophio- 

 derma (Fig. 4) and others. 



The Sea-urchins are very conspicuous; Echinus acutus especially 

 on account of its considerable size. The large yellow ovaries (roe) of 

 Strongylocentrotus and allied forms are eaten in the fish-market raw 

 as we eat oysters. Sphaerechinus (Fig. 7) is of a beautiful violet or 

 reddish-brown colour; and Dorocidaris (Fig. 8) is remarkable on account 

 of the size and thickness of its spines, which are few in number. 



The Sea-cucumbers are very common in the Bay of Naples, and 

 the Aquarium contains abou^ half a dozen kinds; among others the brown 

 Holothuria tubulosa (Fig. 10), attaining the length of one foot; and 



Guide. 4th p<3. 5 



