PROTEOCEPHALUS AMBLOPLITIS LEIDY 183 



SESSIONAL PAPER No. 39b 



cular muscles are here poorly developed. Proceeding from the forked portion 

 of the vesicle are two main longitudinal excretory vessels, each 10;< in diameter, 

 with very thin but distinct walls. These vessels course slightly backwards before 

 passing forward where they connect with a meshwork of vessels of the same size 

 situated in the scolex region; but on account of the invagination of the scolex the 

 latter are directed posteriorly again. Some distance in front of the caudal vesicle the 

 beginnings of the second excretory vessels may be seen in the parenchyma as a 

 very small tube running along the larger vessel on each side and gradually diverging 

 from it as far anteriorly as the latter can be traced. That part of the anterior 

 anastomosis of the excretory vessel mentioned above, which is closely associated 

 with the organ-end and invaginated suckers, is circularly disposed as are the paren- 

 chyma cells, owing to the compression due to invagination; at a later stage 

 when the scolex is permanently everted, they are more loosely arranged. 



Plerocercoid, 2.9 mm. in length, PI. XIX, Fig. 4. 



The plerocercoid of th s size shows practically all of the structures found in 

 the older specimens, so that it will be described somewhat at length. 



Masculature of Scolex. — At a depth of about 15)li from the apex of the 

 scolex general oblique muscle fibres are found coursing from the lateral walls to 

 the dorsal and ventral surfaces, thus forming a rhomboid whose diagonal axes lie 

 in the coronal and sagittal planes of the animal. These also surround the end- 

 organ and its opening quite like similar fibres described by LaRue ('09). As seen in 

 PI. XX, Fig. 7, most of them are attached to the wall of the scolex near the edges 

 of the suckers but some end in the parenchyma before the sucker is reached. 

 They can be traced backward from the tip to a distance of about 150/i beyond which 

 they remain as vestiges only, attached to the indentations between the suckers, 

 PI. XX, Fig. 8; and, furthermore, the farther back one traces them the fewer are 

 those fibres which run between the suckers and the end-organ. This shows that 

 from their points of attachment on the wall of the scolex the fibres curve forward 

 towards the apex, which is well shown in longitudinal sections. 



No "muscle-cross" due to the crossing of rhomboid fibres, with fibres running 

 dorso-ventrally and laterally, connecting opposite structures, as described by La- 

 Rue for P. filaroides, can be made out in this region of the scolex, for here is sit- 

 uated the very large end-organ (PI. XX, Fig. 8) . It is surrounded by a thick m at 

 of circularly arranged fibres which do not appear to run transversely or dorso- 

 ventrally in any part of their course. 



At a depth of 140ju transverse sections of the flared ends of the "diagonal 

 muscle cross" may be seen between the inner walls of the suckers and the wall of 

 the end-organ. Farther on these ends are cut more obliquely and converge towards 

 the end-organ fast diminishing in size as the sections go farther back, until at a level 

 of 230jii the end-organ is just passed and the muscle cross itself is seen very distinctly 

 (PL XX, Fig. 10). In this section the flared ends of the two crossing bundles and 

 their narrowed centres are quite characteristic (LaRue); the fibres are, however, 

 more numerous than those of P. filaroides, according to LaRue' s figure and each 



