ECHINODEEMA. 7 



form the oral vestibule at the moment of metamorphosis, as Mr. Bury (l) has shown, 

 deserts its original median position and moves to the left side of the larva, 

 whilst in the Echinopluteus the original larval ossophagus disappears and a new 

 " oesophagus " is formed on the left side of the larva. This is undoubtedly the 

 more modified development, but in respect of the history of the ccelom the 

 Echinopluteus has retained a far more primitive arrangement than the Auricularia. 

 The fact that the original mouth is retained in Auricularia whilst it is lost and 

 re-formed in Echinopluteus, is paralleled by the fact that the mouth is retained in 

 the Ojihiopluteus larva, but lost and re-formed in the more primitive Bipinnaria larva, 

 yet no one would doubt that an Ophiuroid is derived from an Asteroid. Doubtless 

 in the primitive Echinoids from which the Holothurioidea diverged the larval mouth 

 was retained. 



If, then, we accept provisionally the hypothesis that the Holothurioidea are 

 descended from primitive Echinoidea, a plausible physiological reason can be suggested 

 for their evolution. If ordinary regular sea-urchins be studied in their natural sur- 

 roundings it will be noticed that they frequent by preference crevices among rocks 

 and the vertical faces of rocks, where the numerous tube feet borne by their long 

 radial canals can take hold. Now from habits of climbing the transition to habits of 

 wriggling through narrow crevices and of burrowing is easy, and what little is known 

 of the habits of Holothurioidea points to the conclusion that their normal habits are 

 of this kind. I have myself dug up Synapta inhaerens from its burrows in the mud, 

 and Professor Mitsukuri (8) records that piles of stones are made in order to collect 

 Stichopus japonicus, which is one of the species used for food and known as Trepang.* 

 Sucli habits would require an increase in the muscularity of the body-wall and 

 a disappearance of the corona, and thus the distinctive features of Holothurioid 

 anatomy would appear to have been evolved. 



* Compare F. D. Bennett's account of the habits of the Trepang, ' Whaling Voyage ' (1840), i., p. 175 : 

 " They usually lie exposed in the shallow waters, though we have very often seen them buried in beds of 

 coral sand, their plumy tentacles being alone exposed, and floating in the water above, apparently as a lure 

 for prey. Some may also be observed lying on the rocks, their bodies completely encrusted with coral sand, 

 which may either have been accumulated by a previous burrowing, or thus used as a disguise." F. J. B. 



