ANATOMY OF OPHTOTHEIX VARIABILIS, ETC. 



109 



space is overlain, in the proper position of the animal, by a very 

 thin muscular layer belonging to the interradialis aboralis, and 

 the fibres are attached to the sides of the slit between the jaws, 

 between the separated side mouth-shields, and between these last 

 and the adoral edge of the mouth-shield. 



Considering these lower and superficial muscular fibres as 

 acting in combination with the stouter and higher ones of the 

 interradialis, it follows that contraction or expansion of the in- 

 terbrachial spaces can occur, aborally, to the jaws. The contrac- 

 tion, or the opposite condition, would increase or diminish, as the 

 case might be, the radial direction of the jaws in relation to the 

 jaw-plate, and would push this last inwards or the reverse. The 

 movement of the jaws on the jaw-plate, however slight it might 

 be, would influence the muscles which traverse the jaw-plate and 

 are fixed on the outer or basal part of the teeth (PI. X. fig. 3), 

 as well as those which unite the approximated adoral ends of the 

 mouth-frames — that is to say, the interradiales adorales superiores 

 and inferiores. 



Probably expansion or relaxation of the interbrachial muscles 

 would produce an opposite condition of the tooth-muscles. 

 During expansion, on account of the increased obliquity of the 

 jaws in relation to the jaw-plates, the mouth-opening would 

 slightly enlarge and the muscles of the teeth would be tightened, 

 and the teeth would assume the horizontal position. With con- 

 traction there would be diminution of the space around the mouth, 

 narrowing of the interbrachial areas, and relaxation of the tooth- 

 muscles, accompanied by diminution of the size of the mouth. 



The possible nature of these movements may be gleaned from 

 the following details of the jaw-plates, teeth, and muscles. 



The jaw-plate (torus angularis) is tall, broad and thin (fig. 2), and 

 it projects slightly beyond the line of the sides of the jaws as well 

 as above them and considerably below. It is a very distinct 

 structure in this species, and it is broadest and most projecting 

 inferiorly and rounded there, thence it slopes at the sides up- 

 wards and gradually diminishes in breadth as far as the spot 

 where the true teeth commence. The upper part of the plate 

 which gives attachment to the teeth is not as broad as the inferior 

 portion, but it is rounded off above where it is free. 



The oral surface of the plate is covered on the lower half by the 

 20 or 21 tooth-papillae, and the upper half carries the four teeth. 



AVhen the papillae and teeth are removed, a number of rather 



LINN. JOUBN. — ZOOLOGY, VOL. XXI. 9 



