Anatomy of the Temnopleurida3. 121 



in contact with the crenulation of the boss, so that the 

 manielon is well hidden. As Valentin described in 1841, the 

 joint of the spine and tubercle has three layers — the outer an 

 epithelial, pigmented and ciliated, thin layer, a middle thin 

 and more or less fasciculate layer of muscular fibres, and an 

 inner articular capsule. 



Valentin's researches were made upon species of Echinus 

 with smooth bosses, and as those of Temnopleurus are crenu- 

 late their examination is not devoid of interest, especially as 

 the question of the classificatory value of crenulation is con- 

 stantly arising. 



In Temnopleurus the inner structure surrounding the joint 

 is the articular capsule, which is a white, soft, mass of im- 

 perfectly differentiated matter, with granules, connective 

 tissue, and extremely indistinct fibres. The microscopic ap- 

 pearances are very negative. The white matter adheres to 

 the lower part of the spine, below the milled ring, and to the 

 edge of the cotyloid cavity ; it is firmly adherent to the crenu- 

 lation on the top of the boss of the tubercle. A quantity of 

 the white enclosing substance is spread over the surface of 

 the boss and scrobicule like a flap, but there is some definite 

 connexion between the hard and soft parts. This soft cap- 

 sular structure is very weak, however, and if the outer 

 muscular layer is ruptured, soon gives way to a slight pull or 

 to unusual depression of the top of the spine. The fracture 

 occurs at the point where the free edge of the lower end of 

 the cotyloid cavity is movable upon the crenulation of the 

 boss. Dislocation of the spine then occurs. 



The muscular layer is considerable in height, althougli very 

 thin, and it reaches from its origin around the edge of the 

 scrobicule of a tubercle, upwards to the inferior edge of the 

 milled ring of the spine ; it covers the articular capsule and 

 neither receives fibres from the boss nor from its crenulation. 

 The muscular fibres arise from the connective tissue of the 

 outer edge of the scrobicule, and they are very delicate, 

 separate under the microscope, and exceedingly simple. 

 There are no differentiated structures in them, and the dis- 

 tribution in bundles is not striking, and indeed it does not 

 usually exist. The nervous supply is considerable, beneath 

 the muscular fibres, and the very minute nervous fibres have 

 ganglion-shaped parts upon them*. The delicate layer of 



* The minute anatomy of most of the parts of some species of Echi- 

 nometradse and Echini has been of late worked out with great success, 

 and published by Otto Hamanu (" Beitriige zur Ilistologie der Echino- 

 dermen," Jenaische Zeitschrift, Bd. xxi.. Neue Folge, Bd. xi, p. 87, 



1887). 



