ECHINODERMA. ff 
form the oral vestibule at the moment of metamorphosis, as Mr. Bury (1) has shown, 
deserts its original median position and moves to the left side of the larva, 
whilst in the Echinopluteus the original larval cesophagus disappears and a new 
“esophagus” 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 Awricularia whilst it is lost and 
re-formed in Lehinopluteus, is paralleled by the fact that the mouth is retained in 
the Ophiopluteus 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.* 
Such 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."—I’. J. B. 
