ECHINODERMATA : HOLOTHUROIDEA. 



221 



but not universally, distinct. The generative organs are in the 

 form of long, ramified, caecal tubes, which open externally by 

 a common aperture, situated near the mouth. There is thus no 

 trace of that radial symmetry which is observed in the arrange- 

 ment of the reproductive organs in the other orders of the 

 Echinodermata. 



The skin in the Holothurians is highly contractile, and the 

 body is provided with powerful longitudinal and circular 

 muscles, in compensation for the absence of any rigid integu- 

 mentary skeleton. Many of the Sea-cucumbers, in fact, are 

 endowed with such high contractility that they can eject their 

 internal organs entirely, if injured or alarmed. 



In the family of the Synaptidce. there is no respiratory tree, 

 and the ambulacral tube-feet are wanting; whilst the skin is 

 furnished with calcareous spic- 

 ules of various shapes. The 

 Synaptcz themselves burrow in the 

 mud or sand, and have the skin 

 furnished with innumerable an- 

 chor-shaped spicules (fig. in) 

 attached to special " anchor- 

 plates " in the integument. They 

 often form a kind of protective 

 case or tube of sand-grains ; and 

 they obtain their food by swal- 

 lowing the mud, from which they extract any disseminated 

 nutrient particles. In Chirodota the skin is provided with mi- 

 croscopic calcareous wheels, in the place of anchors. In the 

 OnrinolabidcR, the skin has barbed spicules, and there is no 

 respiratory tree; but these forms differ from the Synaptidce 

 proper in possessing tube-feet. 



The order Holothuroidea is divided into the two following 

 sub-orders : 



Stib-order I. Apneumona. 



No respiratory tree. The ambulacral tube-feet wanting (Synaptidce) , 



or present (Oncinolabidiz}. 

 Sub-order 2. Pneumonophora. 



A respiratory tree. (Ex. Holothuria, Tkyone, Molpadia, Psolus, Cu- 

 i) &c. ) 



Fig. in. Anchor-shaped spicules of 

 Synapta, and the plates to which these 

 are attached. Magnified greatly. 



