118 LECTURE X. 



of the spines, are characterised by more numerous tubercles, and 

 are not perforated. Both kinds of plates are of a pentagonal form, 

 and are arranged each kind in five alternate pairs of vertical 

 rows. The plates of each pair are united together by a zigzag suture, 

 and increase in size as they approach the equator of their living globe. 

 These twenty series of ambulacral and interambulacral plates con- 

 stitute the chief part of the spheroidal skeleton of the Echinus. The 

 large oral aperture is partly occupied by the small irregular oral 

 plates, which have no tubercles or spines, and are suspended in the 

 oral integument, from the middle of which project the points of the 

 five teeth. At the opposite aperture, immediately surrounding the 

 vent, are the small anal plates ; external to these are the five genital 

 or oviducal plates, so called because each is perforated by the duct of 

 an ovarium or testis ; the ocular plates are wedged into the external 

 interspaces of the genital plates, and are pierced near the apex by a 

 very minute pore, which lodges the ocellus and its little nerve. 



One of the genital plates is larger than the rest, and bears a tubercle 

 corresponding with the nucleus or madreporiform tubercle on the 

 back of the star-fish. M. Agassiz, assuming this plate to be at the 

 back part of the Echinus, shewed that the other four genital plates 

 were in symmetrical pairs, and thus discovered the right and left sides 

 of the animal. 



The calcareous constituent of the shell of the Echinus lividus, has 

 the following chemical composition, according to the analysis of 

 Professor Brunner, quoted by Professor Valentin.* 



Carbonate of lime - 96*27 

 Sulphate of lime - 1*53 

 Carbonate of magnesia 0-93 



100-00 



The small anal plates are united together like the oral ones by an 

 extensile and contractile membrane. Both the internal and external 

 surface of the rest of the complicated shell is covered by a similar 

 organised membrane, which likewise extends through all the numerous 

 sutures of the shell. With this explanation of the general structure 

 of the crust of the Echinus we are in a condition to understand the 

 manner of its growth, which otherwise would be a difficult physiolo- 

 gical problem. 



The Echinus maintains nearly the same spheroidal figure from its 

 earliest formation to full maturity ; and, notwithstanding that its soft 



* Mur.ographies D'Echinodcrmes, d' Agassiz. No. I. 1841. 



