126 COMPARATIVE ANATOMY 



anterior division from a fancied analogy to the human stomach, 

 and is unfortunate, for, as the diagram shows, it is the part 

 farthest away from the heart of the crayfish. The roof of the 

 posterior part of the cardiac division, and the whole roof of 

 the pyloric division of the proventriculus, is occupied by a series 

 of peculiarly shaped plates and bars, some calcified, and some 

 merely chitinous, called ossicles, which together form the 

 internal masticatory apparatus or gastric mill of the crayfish. 

 The structure and working of this gastric mill have been 

 explained with such admirable fulness and clearness by the 

 late Professor Huxley in his work on the crayfish, that a brief 

 description oft his piece of mechanism will suffice here, the 

 reader being referred to his memoir for a fuller account. 

 In the roof of the posterior part of the cardiac division is a 

 plate called the cardiac ossicle, whose shape is shown in 

 fig 29, car. To the hinder end of this is attached a short 

 stout bar, the urocardiac ossicle, ur.c, which runs backwards 

 and downwards on the front liihb of the fold or constriction 

 separating the cardiac from the pyloric division of the pro- 

 ventriculus. To either side of the expanded cardiac ossicle 

 is attached a small curved three-cornered piece, the ptero- 

 cardiac ossicle. All these parts being connected by joints, 

 are freely movable on one another, and they form a figure 

 something like a crossbow of which the handle is bent at an 

 angle to the bow itself A somewhat similar crossbow is formed 

 by four other pieces — namely, the pyloric ossicle, situated in the 

 roof of the pyloric division of the proventriculus ; the prepyloric 

 ossicle, passing downwards and forwards from the centre of 

 the pyloric ossicle to articulate with the posterior end of the 

 urocardiac ossicle, and two lateral zygocardiac ossicles, which, 

 fused to either end of the elastic pyloric ossicle, run downwards 

 and forwards in the side walls of the proventriculus, and are 

 articulated to the extremities of the pterocardiac ossicles. 

 The lower end of the prepyloric ossicle is produced into a 

 stout conical tooth, and the inner edge of each zygocardiac 

 ossicle is produced into a thickened ridge bearing a number 

 of large lateral teeth. The whole forms a jointed hexagonal 

 framework, the front and back pieces of which are connected 

 by two jointed rods in such a manner that when these pieces, 

 the cardiac and pyloric ossicles, are approximated, the pre- 

 pyloric rod is bent back under the pyloric ossicle, and the 



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