46 



THE ZOOLOGIST. 



Psohs is the fact that the suckers are confined to the lower 

 (trivial) surface, and that the movement of the creature is, 

 therefore, in a way comjDarable to that of the snail. 



We may now pass to another group in which lungs are like- 

 wise found, but in which there are no special retractor muscles 

 for the tentacles, and these organs, instead of being branched as 

 in Ciicumaria (the Dendrochirotae), are stouter and shield-shaped 

 (Asj)idochirot8e). The best known genus of this group is Holo- 

 thuria, of which, though there are a large number of species, 

 only two are to be found in the British Seas ; H. intestmalis has 

 only been dredged in the Minch, and H. nigra is found in the 

 English Channel off the coasts of Devonshire and Cornwall. 



It is to the latter species — which has various local names, 

 but is best known as the " Nigger" or the " Cotton-spinner" — 

 that we owe our certainty as to the function of the Cuvierian 

 organs. Holothuria nigra has been called the Cotton-spinner 

 from its habit of shooting out from its hinder end white tubes 

 which swell up in the sea-water, and become exceedingly sticky, 

 so that an object — such as a crab — which becomes covered 

 with them finds it exceedingly difficult to set itself free. This 

 pernicious habit causes this Sea-slug to be held in great detes- 

 tation by fishermen. The tubes may be drawn out to twelve 

 times their own length, and at the same time they swell up to 

 seven times their original diameter ; six of these tubes, drawn 

 out to be so thin as to be scarcely visible, are sufficient to hold 

 up a weight of nearly 1000 grains. It is easy to understand the 

 effect such powerful, tenacious, and extensile threads must have 

 on any object which they attack. 



While all these forms are provided both with feet and with 

 lungs there are other Holothurians which depart more widely 

 from the Cucumarian type ; in some lungs, but no suckers, are 

 developed, and in others neither lungs nor suckers, either radial 

 or scattered, are to be found. Of the former of these two groups, 

 of which Molpadia may be taken as an example, we have no 

 representatives in our own seas ; of the latter there are two 

 which are well known — Si/najJta inhcerens and S. digitata. At 

 first sight these creatures appear to be very different from our 

 typical Cucumaria, owing to the fact that the quinqueradiate 

 symmetry is no longer marked by the tube-feet externally or 

 their vessels internally, so that they appear as bilaterally sym- 



