Feb. 26, 18S0] 



NATURE 



595 



The "Gastric Mill" of the Crayfish 



For demonstrating the structure and action of the elaborate 

 giz2ard of the crayfish, which I have found to he usually regarded 

 as a hopeless puzzle, I have constructed, in Prof. Lankester's 

 laboratory at University College, at his suggestion and for the 

 use of his class of practical zoology, a little model, the simplicity 

 of which enables any student to construct one for himself, and 

 thus thoroughly to apprehend the mechanical significance of the 

 apparatus found in the crustacean. 



A description of it will be useful to some of your readers. 



Out of a sheet of good cardboard cut a piece having the shape 

 represented in Fig. 1. Along the lines marked a b, cd, ef hi, 

 in 11, draw a penknife so as to slightly cut into the cardboard, 

 and on the opposite face of the cardboard make similar cuts along 

 the lines g h, k I. Now bend the outstanding pieces, 2, 2, a 



very little downwards, so that they stand at a slight angle to the 

 piece 1. 



K evolve the pieces 9 downwards upon the hinge lines a b, until 

 each of them is brought into the same plane again as the piece 

 6, 5, 6. Fasten, by either gum or a knotted thread, the lower 

 or unseen surface of the shaded bit of 9 fiat against the lower 

 face of the piece 6, *,, 6. Then bend the unshaded portion of 

 each of the ] ieces 9, into a plane at right angles with the shaded 

 portion, using the dotted line, k I, as hinge. These upstanding 

 pieces 9, 9 represent the lateral teeth. 



Now apply gum or a needle and thread to. the shaded piece 

 between 3 and 4, bending the whole piece I, 3, 4, upon the 

 hinge-line j,'//, until the shaded bit lies flat upon the surface 

 of 4, to which it is to be securely joined. Bend back the 

 piece 1, 3, using rf as hinge, until it lies in a plare at right 

 angles to that of 4. The projecting termination of 4, as the 



The apparatus csmplete: at 1 



pieces are now bent and fastened, repre;ents the central tooth 

 of the gastric mil]. Now lend I a little downwards upon 3, 

 using ca'as hinge, and bend 4 upon 5 very much, using m 11 as 

 hinge. Finally, by means of thread or of fine wire, join the 

 perforated corner of the pieces 6, 6, to the corresponding per- 

 forated corners of the pieces 2, 2, right to right and left to left, 

 in such a way that the pieces 2, 2 lie outside the pieces 6, 6, and 

 let the joint consist only of a single thread cr wire, which may 

 act as a pivot for rotation. In order to effect the joining of 

 these pieces, the piece 6, 5, 6 will have to he bent like a bi w, 

 i'.s right and left arms being deflected downwards and inwards. 



The model is now complete. 1 represents the cardiac ossicle 

 or sclerire ; 2, 2, the two pterocardiacs ; 3, the urocardiac ; 4, 

 the prepyloric ; the shaded bit, together with the piece to which 

 it has been affixed, now represents the median tooth, and pro- 

 jects downwards and forwards; 5, the pyloric sclerite ; 6, 6, 

 the right and left zygocardiacs ; whilst 9, 9 represent the hori- 



zontally placed lateral teeth. The anterior and posterior pro- 

 cesses, '8, 8 and 7, 7, represent respectively the anterior and 

 posterior gastric muscles which are affixed in the position indi- 

 cated and to the trm wall of the carapace. If we now pull 

 upon these piece- so as to represent the effect of a muscular con- 

 traction, we shall find that the three teeth come together with a 

 clash, but are again separated and the whole apparatus brought 

 to its original condition by the elasticity of the cardboard. Again 

 and again the clashing of the teeth can be effected by the tension 

 applied at 8 and 9, just as it is in the living crayfish. If the 

 parts re| resenting the three teeth be very carefully adjusted as 

 to size and direction, and be covered with some hard substance, 

 such as sealing-wax (applied after solution in spirit), they may 

 be made really to grind s.ft substances, such as bread, into 

 fragments. 



W. E. Roth 

 University College, Gower Street, February 4 



Modern Chromatics 



In NatuRF, vol. xxi. p. 7S, is a review of a book of mine, 

 " Modern Chromatics," by Silvanus P. Thompson, that contains 

 one or two points that 1 ought perhaps not to allow to ] ass 

 without notice. The statement is made by the reviewer that I 

 claim as mine a certain experiment which w as originally described 

 in England by T. Kore. I find, however, on examination, that 

 Mr. Rose read his paper on this subject before the British Asso- 

 ciation in 1861, while mine was published in September, i860, 

 in the American Jcurnal of Science and Arts. In the same 



review it also stated or implied that I am in error in saying that 

 in blue eyes there is no real blue colouring matter, but that the 

 blue hue is due to the presence of a turbid (or opalescent) medium. 

 Essentially the same statement with details will be found on 

 p. 14 of Helmholtz's " Physiological Optics," also on p. 610 of 

 Dalt. n's "Human Physiology." In his "l'hysiologieder Farben," 

 on p. 95, Briicke remarks ; — " In the most beautiful blue eye 

 there is no trace of any blue colouring matter." 



If any real blue pigment has been discovered in the iris of the 

 human eye, it would interest me to know when and by whom. 



The reviewer also intimates that I am in error in stating that 



