46 ILLINOIS BIOLOGICAL MONOGRAPHS [334 



sists of the slightly enlarged anterior end of the original plerocercoid or larva 

 from which protrude four proboscides, the whole somewhat resembling a hydra 

 and at once reminding one of the Trypanorhyncha. As will be presently seen 

 the latter comparison is a very apt one. Each proboscis consists of a per- 

 manently protruded base or stump, as indicated in figure 43, about 85/x in 

 length and 45 to SSju in diameter, and an eversible proboscis proper having 

 about the same diameter. The former is somewhat conical in shape and thickly 

 set with minute, posteriorly directed cuticular spines which pass on to the 

 neighboring portions of the scolex for a short distance. The whole forms at 

 first sight a continuous tentacle gradually diminishing in size to the pointed 

 end. These tentacles attain a length of 0.35mm., including the base, when 

 fully evaginated, and are directed almost at right angles to the longitudinal 

 axis of the larva, their bases being, however, turned slightly forward (Fig. 43). 

 Within the scolex the tentacles are accommodated in elongated cylindrical 

 muscular sacs which are quite comparable in structure to the bulbs of the 

 Trypanorhyncha. These (the sacs) lie freely in the loose parenchymatous 

 tissue in the diagonal diameters of the region. When the proboscides are 

 invaginated, they have a length of 0.45mm. with a diameter of 0.07, or 0.40 

 by 0.06 when the tentacles are protruded. The walls of the bulb (Fig. 67) are 

 composed of two thick layers of muscles, an outer longitudinal or somewhat 

 oblique — much the heavier of the two — and an inner circular, and a cuticula- 

 like lining, on the inner border of which in transections numerous flattened 

 nuclei appear. The walls are attached to the edge of the stump, and these 

 layers have the same relative arrangement as that of the cuticula and cuticular 

 muscles on the outside of the body, only being in the reverse order. Con- 

 tinuous also with the edge of the stumps are the walls of the proboscis proper, 

 which consist of a thin external layer of cuticula and only feeble cuticular mus- 

 cles. Attached to the wall internally thruout its course are the retractor mus- 

 cles of the proboscis which pass backward and become attached to the posterior 

 end of the bulb. These can be seen best in longitudinal sections where the 

 proboscis is retracted, for then they are closely crowded and much thicker, and 

 their attachment to the inner end of the proboscis is nicely shown. In the 

 retracted condition the latter is, of course, hollow, the narrow cavity often 

 triradiate in transection (Fig. 67) being easily followed into the bulb for about 

 one-third of its length. Closely applied to the cuticula of the tip of the pro- 

 boscis appear in some cases gland-like cells taking the counterstain quite like 

 those behind the bulbs to be described below. They are shown in figure 67. 

 Apart from the structures already described, the contents of the bulbs and 

 consequently of the proboscides to a certain extent, consist of a small amount 

 of loose parenchymatous tissue and what is evidently a good deal of nervous 

 tissue coming into the Dostennr end of the organ. 



Evagination of the proboscides is evidently brought about by the contrac- 

 tion of the muscles in the walls of the bulbs, but the body wall in the vicinity 

 of the latter probably greatly assists since its musculature is well developed. 

 Some distance behind the posterior ends of the bulbs the latter consists of a 



