vin ECHINODERMATA CALCAREOUS RING 403 



ratus, and for its maintenance in the upright position over the oral area. Further 

 investigations, however, must decide whether the bands consist only of elastic fibres, 

 or whether muscle fibres also occur in them. 



The adductor muscles of the teeth (musculi adductores dentium, 10). These 

 are present in five interradial pairs ; they are strongly developed as broad bands. 

 The two muscles of a pair are attached, above, along the outer edge of the arcus of the 

 pair of jaws (pyramid) to which they belong ; and below, along nearly the whole 

 length of the corresponding interambulacral apophysis of the perignathous ring. If 

 these muscles contract, the upper ends of the pair of jaws (or pyramids) are drawn 

 outwards and downwards, forcing the lower ends, with the teeth, inwards, i.e. 

 towards the centre of the mouth. In other words, the externally visible pointed 

 lower ends of the teeth are pressed together. 



The opening muscles of the teeth (musculi abductores dentium sive dilatatores 

 oris, 11). These are five radially arranged pairs of muscles, which run horizontally. 

 The two muscles of each pair are attached on the one side to the inner surface of the 

 ambulacral apophyses (auriculae), and, on the other, to the halves of the jaws nearest 

 them, close to the ends which point downwards. These muscles counteract the 

 adductor muscles ; when they contract, the lower ends of the five pairs of jaws, and 

 with them the tips of the teeth, are moved, centrifugally, towards the auriculae. The 

 teeth move apart, and the mouth opens. 



The intermediate jaw muscles (musculi intermaxillares) connect the apposed 

 lateral surfaces of the five pyramids with one another. The five pyramids close 

 firmly together, when these muscles, which together act like a kind of sphincter, 

 contract. 



The muscles of the forked radii (7) lie on the upturned base of the masticatory 

 apparatus, forming together a pentagonal ring by connecting the five handles of the 

 forks for about half their length. As to the function of these muscles, we can 

 only imagine that they depress the whole masticatory apparatus by their contraction, 

 and thus cause the oral integument to project conically, especially if the adductor 

 muscles of the teeth contract at the same time. It is well known that Echinoids are 

 assisted in locomotion by the bulging forward of the tooth-carrying portion of the 

 oral area, which is supported by the masticatory apparatus. 



In the Clypeastroida, the frequently asymmetrical masticatory apparatus is more 

 or less flattened, usually indeed quite flat. The teeth are not vertical, but slope 

 towards one another quite obliquely, or are even arranged horizontally. The radii 

 are wanting, and the intermediate plates are rudimentary. 



F. The Calcareous Ring- of the Holothupioidea. 



In the Holothurioidea, the oesophagus is surrounded by a circle of 

 ten calcareous skeletal pieces (Fig. 349, 3 and 13), five of which are 

 radial and the other five interradial. This calcareous ring protects 

 the nerve ring at its inner side. For a certain distance it supports 

 the radial water vascular trunks and the tentacular vessels, and may 

 indeed be regarded as the inner skeleton of the oral region of the 

 body. The five longitudinal muscles or pairs of muscles of the body, 

 and, where such are present, the five retractor muscles of the oral 

 region, are attached to this ring, i.e. to its radial portions. The 

 calcareous ring is altogether wanting in the remarkable free-swimming 

 form Pelagothuria (Fig. 224, p. 286). 



