MUSEUM OF COMPAEATIVE 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," c pi, of pentagonal outline, around which are 

 arranged a ring of five plates, pi, closely fitting to the central plate. 

 Outside, or peripherally to these, we see other pentagonal bodies, three 

 below in the right-hand lower comer, and one above adjoining the upper 

 letters, pi. 



In all the peripheral system of plates we have a reticulation of cal- 

 careous nature. 



In the five plates which surround 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 ftie 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. 



