486 MK. A. i). MICHAEL OX THE 



rostrum ; the posterior pair are an approach to tlie perpendicular aud are sliort ; those 

 that are inserted further forward become successively longer and longer and more and 

 more inclined forward, so that the anterior pair are quite long muscles and almost 

 horizontal. All these muscles are inserted into the roof of the pharynx, either directly 

 or each by means of a single tendon; not by numerous short tendons such as attach 

 the corresponding muscles in Thyas petrophilus. 



The transverse muscles also vary consideral^ly from the usual type ; indeed it is 

 evident that some modification would be rendered necessary by the mere fact of the roof 

 of the pharynx being membranous, and consequently very flexible, instead of chitinous 

 and only slightly so. The result of this would be that if the transverse (or occlusor) 

 muscles only approximated the lateral edges of the roof or upj)er half-tube, this action 

 would simply crumple it, and would not drive the central part down upon the lower 

 half-tube or floor. This difficulty is obviated by the following arrangement in Bdella : 

 the transverse bands are not round in section, but are broad flat bands ; each band is 

 arched ; the posterior bands are arched upward, while one of the anterior, which is 

 situated where the oesophagus turns sharply downward, is arched forward and seems on 

 edge in the body. Beneath the anterior band there is a thickened plate, or mass, of 

 tendinous material (figs. 1, 2, 5, 34, pp) in the roof of the pharynx, which doubtless 

 enables the muscle to force it doAvn more effectually. Tlie plate is just where the 

 pharynx merges into the mouth, and it is hard to say whether it is to be considered as 

 wholly a part of the pharynx or wholly or 2:)artly a portion of the mouth ; I incline to 

 the former view. I will, in order to distinguish it, call it the " propharyngeal plate." 

 The plate itself is not a mere straight plate of even thickness ; it consists of two portions — 

 a central plate of thick tendinous material, and a thinner border. The thick central 

 plate is much narrower than the base of the epipharynx, and is considerably wider 

 posteriorly than anteriorly ; its sides have a slightly concave outline when seen from 

 above or below ; its thickness vai'ies in different parts, its ventral surface, adjoining the 

 pharynx, being almost straight, while its dorsal surface curves rapidly upward ; thus the 

 thickest part of the plate is almost at its posterior end ; this end is sharply truncated 

 and has a perpendicular wall. Immediately behind this wall the second transverse band 

 of pharyngeal muscles (constrictors) is placed nearly on edge, instead of having its broad 

 side downward like the other muscles of the same series ; like them it is arched but has 

 its concavity forward, thus when it contracts it must force the propharyngeal plate and 

 epipharynx somewhat forward. One fasciculus of distensor (levator) muscles (fig. 5) 

 on each side of the median line is inserted by a very short tendon into the supero- 

 posterior angle of this thick central part of the propharyngeal plate, and another pair on 

 its dorsal surface about a quarter of its length behind the anterior end. The posterior 

 of these tendons may be traced some distance in the substance of the plate. The border 

 is of tissue similar to the central portion, but much thinnei* ; it runs along the sides and 

 rear, but not along the front, where the propliaryngeal plate joins the epipharynx ; it 

 makes the whole plate form an oblong slightly wider than the epipharynx ; the border 

 slopes slightly downward, and the portions of it at the sides of the central plate have a 

 number of diagonal parallel ridges (or folds forming ridges) running backward and 



