356 ECHINODERMS OF THE BRITISH ISLES 



in the ground, or attached to stones, etc., or creep over the 

 bottom, mainly by means of the contractions of the body 

 musculature. Especially in the transparent Synaptids the 

 peristaltic movements of the body wall are seen very beautifully, 

 like waves proceeding gently from one end of the body to the 

 other. The tube-feet are not very important as organs of move- 

 ment ; they serve rather for attachment. In Synaptids the 

 calcareous corpuscles are of importance as organs of movement. 

 Some forms {Bathyplofes natans and, probably, several deep-sea 

 forms, e.g. Psychropotes) can swim, with bending, up-and-down 

 movements. A few are wholly pelagic {Pelagothuria, Plankto- 

 thuria). Some Holothurians {Pseudostichopus, Meseres) cover 

 their skin with foreign bodies, sand, shells, sponge spicules. The 

 sea-cucumbers of the British seas are, with a few exceptions {e.g. 

 Cucumaria frondosa, Holothuria Forskali), small forms ; in tropical 

 seas several good-sized forms occur, about a metre long,' e.g. some 

 Synaptids which live in quite shallow water, on coral reef flats 

 or creeping among the sea-grass. The food of the sea-cucumbers 

 consists of bottom material, which is shovelled into the mouth 

 by means of the tentacles (Aspidochirotes, Molpadids, Synaptids), 

 or of plankton and detritus, which is caught by the tentacles 

 (Dendrochirotes) ; they lie with their tentacles widely extended 

 and then alternately bend them down and put them into the 

 mouth, thus sucking off the organisms or detritus particles which 

 stick to the tentacles. 



As in other echinoderms the power of regeneration is very 

 great. Most Holothurians, especially the larger forms, usually 

 contract very strongly under unfavourable circumstances and 

 may then expel all their inner organs through the anus or through 

 a rupture in the skin. Such eviscerated specimens are able to 

 regenerate again all their organs in the course of some time ; in 

 Stichopus tremulus it takes 2-3 weeks, in a tropical form it has 

 been observed that the regeneration is completed in 9 days. 

 The anterior end, with tentacles and calcareous ring, may also 

 regenerate, at least in some Dendrochirotes. In the Synaptids, 

 which, under unfavourable circumstances, are apt to constrict 

 their body in various places, and break to pieces, the anterior 

 end will regenerate the rest of the body, while the other parts 

 are not able to do so. Propagation by means of autotomy occurs 

 normally in a few forms, e.g. Cucumaria lactea and Psolus valvatus. 

 Budding (a new anterior end forming on the side of the body, 

 beneath the original anterior end) has been observed in Cucumaria 

 lactea. In a small tropical sea-cucumber {Thyone gibbera Sel.) 



