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Show 182 ME. A. D. MICHAEL ON AN [Mar. 5, water gutters placed round the roofs of houses ; they both have the convex surface downward and the upper rests upon and within the under; the edges, not the ends, are joined in a slightly flexible manner, and the whole upper wall is more or less flexible, elastic, and movable, while the lower wall is stiffer and more fixed. Thus, when the sucking-muscles (mlp.) contract and the upper wall is raised, a crescent-shaped lumen is formed in the pharynx (or throat). The pharynx is contracted again partly probably by its own elasticity, but chiefly by the transverse muscles (Henkin's swallowing muscles) which run straight across from one edge of the half-tubes to the other (his fig. 5, m y figs. 23, 27, mop.), one band of transverse muscle alternating with each band of perpendicular muscle. Henkin was not the first to describe this arrangement; Croneberg drew and described it most exactly four years previously in Eylais; but I have above referred to Henkin rather than to Croneberg because the former gives a sagittal as well as a transverse section, which makes reference easier, and Croneberg shows the pharynx and the oesophagus in the same transverse section, which I do not quite understand ; moreover, Croneberg's paper is in Eussian. A similar construction was given by MacLeod in 1884 for Trombidium, Hydrachna, and Erythreeus 2 Coming now to the present species, Thyas petrophilus, I find the pharyngeal arrangement to agree entirely with Croneberg's and Henkin's descriptions, and not at all with Schaub's; I have examined it with great care by sections in every direction and by dissections, and I cannot find a trace of Schaub's thin tube, his " true throat," while the food certainly seems to pass between the two chitinous floors as Henkin says (figs. 23, 27, ph.). The long perpendicular muscles, which I will call the dilatores pharyngis muscles (figs. 23, 27, mlp.), raise the roof of the pharnyx (Schaub's ch. 2), principally in the median line along which they are attached, and the food rushes iu between it and the chitinous floor; the roof by the action of the muscles having been separated from the floor and at its posterior end become continuous with the upper wall (or roof) of the oesophagus, which seems above it when the dilator muscles are not in action. The valve before described now closes the anterior end of the pharynx; tbe contraction of tbe transverse muscles (depressores tecti pharyngis or contractores pharyngis) brings the roof and floor close together and drives the food into the oesophagus, its return from which is prevented by a valve. Such, at least, is m y reading of the action of the parts; at all events I do not think that there can be any doubt that the passage for the food from the mouth to the oesophagus in Thyas petrophilus is below, not above, tbe chitinous plate, to which what I call the "dilatores pharyngis" are attached. I may say that I have carefully examined similar parts in two or three species of Trombidium and other allied creatures, and in every instance have found the construction the same. I have also examined them in 1 "La structure de l'intestin anterieur des Arachnides," Bull. Acad. E. de Belgique, 1884. nos. 9, 10, |