INTERNAL ANATOMY 01' BUELLA. 485 



The mouth-parts have ah'eady been described. 



The Pharynx is of the usual Acarine type, but exhibits a lew special features; it is 

 well, ill the first place, shortly to state what that type is. The pharynx is the great 

 sucking-organ in all Acari, and in most of those which live by suction, as practically all 

 jjredatory Acari do, but especially in the Trombidium-groui^, the pharynx consists of two 

 chitinous half-tubes, like gutter-pipes, concave upward, the upper fitting closely upon 

 find within the lower. The lower is really the continuation of the maxillary lip and 

 forms the fioor of the pharynx : the upper is the roof of the pharynx ; it is by the 

 movements of this roof that suction is ett'ected. The following is the mechanism : bands 

 of perpendicular muscle arise from the underside of the roof of the rostrum and are 

 inserted on the upperside of the roof of the pharynx, which is raised when they 

 contract : the food rushes into the partial vacuum thus created ; the anterior end is 

 closed by a valve which prevents the food returning to the mouth. Between each band 

 of perpendicular muscles there is a transverse muscle, usually round, running straio"ht 

 across the upper tube I'rom one edge to the other ; when these mtiscles contract the 

 edges are drawn together and the central parts of the vq^per half-tube are driven down 

 upon the lower one, thus forcing the food on into the oesophagus. Even such Acari as 

 can consume solid food, as the Tyroglyphida? and the Oribatidae, usually have the 

 pharynx constructed upon some modification of tliis plan — which, however, is somew hat 

 varied in the case of the Gamasidie and others, but the modifications are more of detail, 

 in the form of the lumen and the conseqttent arrangement of the muscles, than of general 

 principle. In most Acari the muscle-bands are numerous, and the " jierpendicular " ones 

 or levatores are almost perpendicular. 



In Bdella the first modification of the general arrangement which is observed is that 

 the roof of the pharynx is only slightly chitinized, and is indeed almost membranous ; 

 the result of this is that instead of tlie whole roof rising in response to the action of the 

 perpendicular muscles, each muscle only raises the part into which it is inserted ; the 

 muscles apparently contract in succession from before backward : thus an undulatorv 

 motion is caused, Avhich swiftly carries the food back to the oesoj^hagus ; the anterior 

 perpendicular muscles relaxing, while those posterior to them are contracting, allow the 

 anterior part of the roof of the pharynx to descend ujion the floor, forming the valve, 

 Avhich is differently constructed in other allied families. 



The next variation from the tisual type is that, instead of the numerous bands of 

 small muscles commonly fotmd in the pharynx of Acarina, Bdella possesses only a few 

 bauds, which are necessarily larger. In Bdella Basteri there are six ^'dk^% of perpen- 

 dicular muscles and six transverse muscle-bands, all considerably larger than in most 

 iamilies. 



I have preserved the name of perpendicular nuiscles for the '■'levator tectl pharrjngls" 

 or " dlstensor jihari/nffis " muscles (figs. 2, 3, ml^j) because the name has been so frequently 

 used in relation to other families ; but the next variation from the usual type which has to 

 be noticed is that in Bdella, doubtless as the result of the extreme length of the rostrum 

 combined with Ihe small number of pharyngeal muscles, the levator muscles are not 

 really perpendicular : they all arise from the hinder and stronger part of the roof of the 



SECOND SERIES. — ZOOLOGY, VOL. VI 66 



