182 MR. A. D. MICHAEL OTS 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 slightl)' flexible 

 manner, and the whole upper wall is more or less flexible, elastic, 

 and movable, while tlie lower wall is stiffer and more fixed. 

 Thus, when the sucking-muscles (mlj>.) 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 swallow- 

 ing muscles) which rini straight across from one edge of the half- 

 tubes to the other (his fig. 5, my 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, Hydraclma, and Eryilirceus \ 



Coming now to the present species, Thyas petrophihis, I find 

 the pharyngeal arrangement to agree entirely with Croneberg's and 

 Henkin's descriptions, and not at all with Schaub's ; I have ex- 

 amined 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 w-ill call the dilatores pharyugis 

 muscles (figs. 23, 27, mlp.), raise the roof of the pharnyx (Schaub's 

 cli. 2), principally in the median line along which they are attached, 

 and the food rushes in 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 ; the contraction of the 

 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 Mhich is prevented by a 

 valve. Such, at least, is my 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 

 petrophUus is below, not above, the chitinous plate, to which what 1 

 call the "dilatores pharyngis" are attached. I may say that I have 

 carefully examined similar parts in two or three species of Trom- 

 bidium and other allied creatures, and in every instance have 

 found the construction the same. I have also examined them in 



^ "La structure de I'intestin anterieur des Arachnides," Bull. Acad. R. de 

 Belgique, 1884, nos. 9, 10. 



