236 B. W. MACBEIDB. 



PI. XX, fig. 137j shows the character of the wall of the prae- 

 oral lobe. The peritoneal cells have developed fine muscular 

 tails {muse, larv.), and it is perfectly apparent to anyone looking 

 at sections of a number of larvae 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 

 coelom by a great accumulation of the fluid of the blastocoele, 

 or rather (as we must conclude from observations which have 

 been made on other Echinoderms) the blastocoelic semi-fluid 

 jelly. In fig. 137 we see some fine fibrils traversing the blasto- 

 cojle; these, so far as 1 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 PI. XX, 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 larvae 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 prte- 

 oral lobe, the larval organ being applied to the substratum. 



In the first case, that of truly sessile larvae, if one attempted 

 to remove them with a pipette, one failed to move them unless 

 very strong suction was applied or tliey were displaced by a 

 needle; but once displaced they Avere 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 



