10 COMPARATIVE ANATOMY. 



16. The oral skeleton, whole and separated into its com- 

 ponent parts. 



17. The lower half of an Echinus (E. sphcera), prepared 

 to shew the mouth, and the muscles that move the jaws. 



"The apparatus consists of five hollow wedge-shaped cal- 

 careous pieces, the ' alveoli,' each of which is composed of two 

 halves united together in the mesial line, while each half again 

 consists of a superior epiphysis and an inferior principal portion. 

 Each ' alveolus ' serves as the socket to a long tooth shaped like 

 the incisor of a rodent, harder externally than internally, so as 

 always to develop a sharp edge with wear, and constantly grow- 

 ing from its upper extremity. The five alveoli, fitted together, 

 form a cone, whose applied surfaces are united by strong trans- 

 verse muscular fibres, while superiorly the epiphyses of each 

 pair of alveoli are connected by long radial pieces, the ' rotulae.' 

 To the inner extremity of each 'rotula' finally a slender 

 arcuated rod presenting indications of a division in the 

 middle of its length is articulated, and running outwards 

 parallel to the ' rotula' terminates in a free bifurcated ex- 

 tremity. This is the 'radius' or ' compass' of Valentin. Al- 

 together the ' lantern' consists of twenty principal pieces, five 

 teeth, five ' alveoli,' five ' rotulae,' or ' falces,' and five ' radii ' or 

 'compasses:' of which the alveoli are again divisible into four 

 pieces each, and the radii into two: making a total of forty 

 pieces. In their normal position it must be remembered that 

 the alveoli and teeth are inter- ambulacral, while the 'radii' and 

 'rotulae' are ambulacral. Besides the inter-alveolar muscles 

 this complex apparatus has protractor muscles arising from the 

 inter-ambulacral region of the oral edge of the 'corona/ and 

 inserted into the upper part of the alveoli; slender oblique 

 muscles, with a similar origin, but inserted into the radii ; trans- 

 verse muscles connecting the radii together; and retractor 

 muscles arising from the arches of the 'auriculae,' and inserted 

 into the oral ends of the 'alveoli.'" 



Huxley, Medical Times and Gazette, 1856, vol. xiii. p. 587. 



The next three preparations illustrate the anatomy of the 

 Holothuroidea. In each a blue rod has been placed in the 

 znouth : a red rod in the anus. 



18. A Great Sea-Cucumber (Cucumaria frondosa), verti- 



