310 Mil. A. D. MICHAEL ON THE VAEIATIONS IN THE 



part and run forward ; and two pairs of similar but larger caeca spring from the lateral 

 posterior part and run backward : these are arranged one aliove the other ; the upper 

 pair are the longer : they usually agree in both sexes. 



In R<^mogmnasus horridus the female agrees with the above description, but the male 

 is entirely without the lower pair of hind caeca (PL XXXV. fig. 72). 



In L(pl(ips cimeifer the anterior median ca3cum is bifid at its anterior extremity, each 

 half curling slightly outward in a rather singular hood-like m.anner ; the anterior lateral 

 ca?ca also arc recurved at their distal ends in a form wliich I have not seen elsewhere ; 

 the hind central caecum is very wide. I have omitted the drawing which I had made 

 of this canal in order to relieve the already overcrowded Plates. 



An interesting feature is found in the pharynx of Ilcemogamasus horridus. It is 

 well known that in predatory Acarina, and indeed in most that live by suction, the 

 pharynx is the sucking-organ, and that it is provided with a special set of muscles 

 beautifully adapted to dilate and contract it. This, which has been well described by 

 Henking *, MacLeod f, and others, is not confined to the Acarina, but is found also in 

 many other Arachnida. Winkler also, in the paper so often referred to above, describes it 

 well in Qamasus crassipes, and shows there that the lumen of the pharynx is an 

 upright oblong with two branches at its upper end ; he also mentions that its walls 

 are somewhat chitinized ; he gives excellent delineations of the musculation. 



In dissecting Hcemogamasus horridus I was surprised to find that the development 

 in the same direction had gone much further, and that there was au actual chitinized 

 skeleton to the pharynx, forming a closed chitinous organ of a very definite shape 

 and highly elastic ; this is delineated by fig. 23 (PI. XXXII.), from which it will be seen 

 that the whole structure is lanceolate and comes almost to a point posteriorly, where the 

 CESophagus arises from it ; and is truncated anteriorly, where it joins the mouth. The 

 organ is tricarinatc, i. e., if three carina3 of birds' sterna were hollow and open at 

 their proximal edges, and were placed with these edges together so as to form one 

 common lumen, and to diverge from one anotlier at equal angles, a very fair model 

 of this pharyngeal skeleton would be obtained ; but it is all one piece, without any 

 break or space between the carimie, which are firmly anchylosed to each other. 

 Thus the lumen in a transverse section of the organ looks like a triradiate sponge- 

 spiculc (fig. 73, p^i)- It will be seen from the same figure that one carina goes per- 

 iiendieularly downward, that the two others branch from it like the arms of the letter 

 y and that the large distensor muscles are attached to the side-surfaces of all tlie 

 carina^ ; while the much smaller constrictors pass from one edge to the other, and are 

 doul)tless aided by the elasticity of the chitin. In spite of the small size it is quite 

 possible to dissect tliis organ out, and fig. 23 was drawn from such a preparation, 

 which is still in my possession. 



It may be mentioned that in Hceinoyauiasiis horridus and II. hirsulus the two great 



* "Beitriio'e zur Auatomie, EntwiokUingsgeschiclito uiid Biologic von Tromhidiiim fidiijinusum, Ilcrm.," Zeitselir. f. 

 wiss. Zool. Bd. xxxvii. (1883) pp. 501), 570. 



t " La structure de Fintestin anti'rieur dcs Arachnidos," Bull. Acad. B. d. Belg. 18S4. 



