MUSEUM OF COMPARATIVE ZOOLOGY. 137 
Formation of the Young Hchinarachnius. 
The growth of the young Echinarachnius from its pluteus is not easy 
to trace on account of the condensation of pigment upon its walls as it 
matures. This formation of pigment renders it very difficult to study 
the sequence of the appearance of the plates, and obscures the internal 
changes which accompany the maturation of the larva into the adult. 
The contour of the young sand-dollar after it absorbs the pluteus is very 
different from that of the adult. No one would recognize both as be- 
longing to one and the same Echinoid. The whole of the pluteus is 
absorbed into the growing Echinarachnius. 
A vesicle, the vaso-peritoneal vesicle, on the left hand side of the 
stomach (see figures) appears in the very earliest stages of the growth 
of the sea-urchin from the pluteus to enlarge, and was observed to have 
the form of a retort-shaped structure, with an external opening on the 
dorsal side of the body, near the posterior arms, Pl. VII. fig. 3. It was 
not possible for me to determine whether the left “ water-tube” sends 
out a prolongation which forces its way to the surface, opening through 
a dorsal pore, as A. Agassiz has described in Strongylocentrotus, or 
not. In the earliest stage in which I began to study the growth of the 
young sand-dollar, the dorsal opening had already formed, communi- 
cating through a tubular body with the water-tube. Consequently, 
the growth of the tube through the body was not observed or studied. 
In the pluteus in which this external opening had formed, the arms of 
the pluteus were all of the same length, and consequently the pluteus 
was regarded as adult. In the pluteus of Strongylocentrotus, accord- 
ing to A. Agassiz, the young sea-urchin first appears in a young or im- 
mature pluteus, in which the arms are not of the same length, judging 
from his fig. 52, in “ Revision of the Echini, Embryology,” p. 717. In 
this figure the antero-internal arms had not begun to push out from 
the oral lobe, and the antero-lateral rods were just formed. This plu- 
teus appears to be immature as far as the appendages go, since they 
are not fully formed. The beginning of the young Echinarachnius on 
the left water-tube was not traced in a pluteus as young as this pluteus 
of Strongylocentrotus. 
Balfour * in his account of this figure (fig. 52) gives an interpretation 
to the structure, ¢, different from A. Agassiz. The latter author says, 
“On the left water-tube we notice a very prominent loop, ¢, which, from 
* Op. cit. pp. 472, 473. 
