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, PI. VIII. figs. 15, 16, of the young Echinarachnius 

 here considered, were taken by dredging in the shallow w^aters 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 (PI. 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. 



