THE DEVELOPMENT OF ASTERINA GIBBOSA. 357 
adult plane makes an angle of about 70° or more with the 
larval plane; but without any very serious error, it may be 
regarded, for purposes of description, as at right angles to it: 
thus the direction right to left, according to the larval plane, 
becomes aboral to oral according to the adult plane, and dorsal 
to ventral according to the larval plane is nearly parallel to 
the adult plane. Here I may remark that the words “ dorsal” 
and “ventral” will only be used with reference to the larval 
plane; in speaking of the adult plane the words “ oral” and 
“ aboral”’ will be used. 
Pl. 18, figs. 12 and 13, show the appearance of a larva which 
has only been fixed for a short time. On the left side we see 
that the hydroceele lobes have become visible externally, since 
they have raised the ectoderm into protrusions which, as we 
shall find, are the rudiments of sensory terminal tentacles of 
the radial water-vascular canals. Outside the curve of these 
rudiments is another set of protrusions, also arranged in an 
open curve. These are the rudiments of the arms: they are 
all, as we shall see, outgrowths of the left posterior ccelom, and 
their primary function is to form supports for the lobes of the 
hydrocele, to which they later become apposed. The con- 
striction of the przoral lobe or stalk from the body proper is 
hardly as yet marked, but the rounded appearance of the dorsal 
and ventral outgrowths of the preoral lobe is to be noticed. 
This is due to the disappearance of the larval organ, the opposite 
‘sides of which become approximated to each other and wrinkled, 
and then broken up, portions of the organ becoming invaginated 
into the interior and destroyed by histolysis. The appearance 
of the remnants of it at this stage gave Ludwig the impression 
that one had to do with the outgrowth of a series of protrusions 
homologous to the adhesive disc. Thisis, of course, a mistake; 
the adhesive disc remains single and unaltered to the end of 
the metamorphosis. This well-marked phase of development 
we may call Stage E. PI. 20, figs. 48 to 50, are taken from a 
larva of this age; fig. 48 is of course the most dorsal section 
(see explanation of plates). In fig. 50 we notice the great 
srowth of the left hydroceele, lobe 3 reaching nearly to the 
