200 LECTURE X. 



which constitute a hollow spheroid, having a large aperture at each 

 wy/i ,1^///. pole, where the first four kinds of 



^,v^vj <ru:^^<j^// plates are situated, ihe ambula- 



cral plates (a) are perforated for 

 the passage of the tubular feet, the 

 parallel rows of which intercept 

 and overshadow spaces compared 

 by Linnjeus to avenues or ambula- 

 cra ; these plates likewise support 

 spines. The interambulacral plates, 

 {i) which support a greater num- 

 ^^^"""^' ber 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 ambulacra! 

 and interambulacral plates constitute 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, imme- 

 diately 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, showed 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 Pro- 

 fessor Brunner, quoted by Professor Valentin.* 



Carbonate of lime - - 96-27 



Sulphate of lime - - 1 '53 



Carbonate of maofnesia - 0-93 



100-00 

 * CLXVII. 



