MUSEUM OF COMPARATIVE ZOOLOGY. 143 
identified, on account of pigmentation, although a single opening, ma, 
which is thought to be the madreporic opening, was clearly observed. 
This interpretation of the opening, ma, is conjectural ; for, with the 
exception of the single fact that it occupies the same position as the 
opening, ma, of previous stages, there is nothing to show that it is 
the madreporic opening. Its communication with the water system 
could not be traced. 
The oldest stages, Pl. VIII. figs. 15, 16, of the young Echinarachnius 
here considered, were taken by dredging in the shallow waters on a 
sandy bottom, where these Echinoderms live. The dredge brought up 
a large number of very small sand cakes which were free in its meshes, 
while many of the younger specimens were washed out of the sand and 
“roots” of Laminaria from the bottom. These young Echinarachnii 
are regarded as developed from plutei hatched the past summer. They 
were dredged near the end of September. A young Echinarachnius, 
older than that here (Pl. VIII. fig. 15) described, is figured by A. 
Agassiz.* My figures represent stages between that which he has 
given and the young Echinarachnius, just after it has absorbed the 
pluteus. The form of the young sea-urchin in this stage is spherical, 
elongated, plump, more like a Spatangoid or some “ round sea-urchin ” 
than a Clypeastroid. A. Agassiz has compared it to that of the genus 
Echinometra. The larger diameter is 1 mm.; the smaller.8 mm. The 
young are almost completely opaque, on account of the formation of 
spines, pigment, calcareous rods, and plates. 
The spines are relatively larger and more prominent than in the 
adult. In many of these structures the superficial layer of the spine 
closely hugs its calcareous centre forming the shank, while in others, 
mostly younger, the thickness of the outer transparent layer is percep- 
tible in lateral profile. The sea-urchin, when seen from the abactinal 
area, is found to be oblong, a diameter passing through the anus being a 
third longer than that at right angles to it. The anus is slightly excen- 
tric, and has the form of a crescentic slit, which is formed by a circular 
plate, ‘‘centrale,” ap, almost closing the circular opening, leaving a 
crescentic orifice with concavity towards the apex. The ambulacral 
areas are distinguishable from the interambulacral at the apex of the 
body, while near the periphery of the test (seen from the aboral region) 
their discovery and separation is more difficult. The spines, sp, around 
the rim of the body are large and long. The ambulacral feet, am, are 
widely extended. 
* Loc. cit. 
