354 E. W. MACBRIDE. 
P|. 27, fig. 187, shows the character of the wall of the pre- 
oral lobe. The peritoneal cells have developed fine muscular 
tails (musc.larv.), and it is perfectly apparent to anyone looking 
at sections of a number of larve that it is the peritoneum which 
is the active agent in contraction. The ectoderm is often 
wrinkled (fig. 38), but the peritoneum never, though its cells 
vary in shape from cylindrical to flattened according to the 
state of contraction ; thus in some cases the peritoneal cells on 
the left side will be cylindrical, those on the right side flattened. 
The coelomic wall is in this case short and straight on the one 
side, and on the other bulged in to the lumen of the anterior 
celom by a great accumulation of the fluid of the blastoceele, 
or rather (as we must conclude from observations which have 
been made on other Echinoderms) the blastoccelic semi-fluid 
jelly. In fig. 137 we see some fine fibrils traversing the blasto- 
coele; these, so faras I can make out, are not protoplasmic, but 
of skeletal nature—of the same nature, that is, as the adult 
fibrous tissue. 
The Metamorphosis. 
On the eighth day the larva fixes itself by the adhesive disc 
by means of a thin secretion of mucilage (see Pl. 27, fig. 136, 
which represents a much later stage), and remains fixed 
during the whole of the metamorphosis. I had the 
opportunity of observing this in Plymouth in 1893 and in 
Jersey in 1894, and it was most instructive to observe the 
difference between the larve which had thus definitely become 
sessile and those which, being still able to move, had attached 
themselves by the cupping action of the muscles of the pre- 
oral lobe, the larval organ being applied to the substratum. 
In the first case, that of truly sessile larve, if one attempted 
to remove them with a pipette, one failed to move them unless 
very strong suction was applied or they were displaced by a 
needle; but once displaced they were perfectly helpless, those 
even which had to all appearance almost completed the meta- 
morphosis being unable to use their tube-feet (which as yet 
were rudimentary) ; they could do nothing but feebly rotate by 
