MUSEUM OF COMPARATIVE ZOOLOGY. 141 
the left-hand side we see the plates of the test of the future sea-urchin. 
In the middle of the figure, a little to one side (left), we notice a cen- 
tral plate, “centrale,” ¢ pl, of pentagonal outline, around which are 
arranged a ring of five plates, p/, closely fitting to the central plate. 
Outside, or peripherally to these, we see other pentagonal bodies, three 
below in the right-hand lower corner, and one above adjoining the upper 
letters, pl, 
In all the peripheral system of plates we have a reticulation of cal- 
careous nature. 
In the five plates which snrround the central plate we have two 
kinds of calcification, one of which forms plates of the test, the other 
probably the spines of the plate. The calcareous deposit of the plate 
forms an irregular network or reticulation of no regular form, while 
the calcareous deposit of the spine has a circular wheel-shaped or stel- 
late form, from the rim of which there spring prolongations, in our 
figure drawn in a fan shape. The circular portion is the base of the 
spine ; the fan-shaped continuations or extensions, the beginning of the 
shaft. In some of the plates which are more peripherally arranged as 
regards the five plates described, we find fan-shaped calcareous forma- 
tions, and no reticulated or lace-formed calcifications corresponding to 
them. 
In the stage which we are considering, the centre of calcification 
which is supposed to form the plate of the test (reticulated calcification) 
and that which later forms the spine (stellate calcification) are not 
joined. 
The development of the spine in sea-urchins and starfishes has been 
traced by A. Agassiz. He says:* “The shell of a sea-urchin is made 
up of an irregular network of limestone cells, which makes its appear- 
ance in the early pluteus stage; with increasing size this network 
becomes closed at certain points, and sends off upright shanks, which 
little by little form very irregular fan-shaped spines. In our common 
sea-urchins these spines are immovable, forming at that stage part of 
the test itself. As the spines grow they become more pointed, but are 
still immovable. In somewhat more advanced stages a slight constric- 
tion is formed at the base of the spine, and very soon after that, below 
the constriction, a tubercle is formed, upon which the spine is articu- 
lated, and is then capable of a certain amount of motion, etc.” (I have 
omitted his reference to plates and figures.) It would seem, then, look- 
* Revision of the Echini, pp. 667-669. 
