

OF URCHINS IN GENERAL. 233 



are therefore fixed with respect to their points of 

 attachment only. We must then suppose the 

 animal placed in certain positions, in order that 

 the parts described may be more easily found. 



" Let us suppose the Urchin rolled up, as in the 

 position the animal assumes for defence. All the 

 body is then enveloped under the skin, by an oval- 

 shaped sac, composed of fleshy and concentric 

 fibres. These fibres adhere closely to the skin, and 

 even to the root of the spines which cover it, and 

 it is difficult to detach them with instruments. The 

 fleshy purse they form is thickest at the margin of 

 its aperture, which corresponds to the belly, at 

 which place they form a kind of sphincter, or 

 muscle, with orbicular fibres. 



" When the body of the Urchin is elongated, as 

 in standing or running, the figure of this muscle is 

 completely changed. It is situated on the animal's 

 back, where it forms an oval, the middle part of 

 which is very thin, but the circular margin con- 

 siderably thicker, and more elevated. Several ac- 

 cessary muscles arise from the different points of 

 the margin. 



" Towards the head, or at the anterior extremity 

 of the oval, we observe two pairs of accessary 

 muscles; one has its origin in the middle line, and 

 is inserted into the bones of the nose; the other, 

 which arises more externally, appears to be con- 

 founded with the exterior orbicular fibres, and is 

 inserted anteriorly into the lateral parts of the nose 



and 



