THE ZOOLOGIST, 



THIRD SERIES. 



Vol. XL] FEBRUARY, 1887. [No. J 22. 



HOLOTHURIANS OR SEA SLUGS. 

 By Prof. F. Jeffrey Befx, M.A. 



We are so much in the habit of associating hardness of the 

 external parts with the Sea-urchin, the Starfish, or the Brittle- 

 star that it is not at first sight easy to believe that the soft- 

 boclied Sea-slug belongs to the same great division of the Animal 

 Kingdom as the forms just named. There are, however, very 

 good reasons for placing the Sea-slugs or Holothurians (in the 

 same division of the Echinodermata with the Urchins (Echinoids), 

 the Starfishes (Asteroids), or the Brittle- stars (Ophiuroids). 



If we put our knowledge of the fact that a number of these 

 echinoderms have hard outer skins into a generahsed form, we 

 should say that the Echinodermata had deposits of lime-salts in 

 their integument ; this is as true of nearly all Holothurians as 

 it is of other members of the group ; the striking difl'erence is 

 that these deposits in the Sea-slug are not, as a rule, continuous ; 

 they do not form a compact test, the parts of which fit close to 

 one another as they do, for example, in the Urchin, nor do they 

 form a lattice- work, as in the Starfish. As an ordinary rule, 

 the calcareous deposits are scattered spicules, which may be 

 rod-like, cup-shaped, or discoid, like wheels or anchors ; some- 

 times, indeed, they form, as in Psolus fabricii, a continuous 

 arrangement of tile-like scales on the dome-shaped upper surface; 

 and, on the other hand, they sometimes are altogether absent, 

 or disappear with the advance of age ; it is obvious that they 

 must be reduced to a minimum in those species which are 



ZOOLOGIST. FKB. 1887. ^ 



