FISH ENTOZOA FROM YELLOWSTONE PARK. 649 



larger speciiuens is rather sharply marked off from the body for a dis- 

 tance of 3 mm. or more. The anterior end is somewhat lighter colored 

 than the body and suggests the head of some species of Dibothrium, 

 but there are no bothria. The body, especially toward the posterior 

 end, is yellowish-white. 



ANATOMY. 



The sections upon which the following account of the anatomy is 

 based were made from specimens stained in toto in borax carmine. 



Structure of the head. — Upon examining transverse and longitudinal 

 sections of the head it is seen to consist of somewhat spongy tissue, in 

 which tliere are a few longitudinal muscular and connective libers and 

 numerous jirotoplasmic granules and granular masses contained in a 

 mesh of transverse connective fibers. There are also numerous small 

 o^jen spaces shown in the sections which are doubtless the fine and 

 irregular branchings of the vessels of the water-vascular system. 

 Figs. 9 and 10 show the structure of the head of a small specimen. In 

 them it is seen that the cuticular layer is very thin and the subcutic- 

 ular granulo-fibrous layer, which is clearly defined towaid the middle 

 of the body, is but faintly foreshadowed. The longitudinal muscular 

 fibers are not collected into any definite area, but are distributed 

 pretty uniformly among the other tissues. There is no indication of 

 layers of any kind. If the entire section, from which Fig. 10 was 

 sketched, had been figured it would have shown no peculiarities of 

 structure that are not shown in the small portion which is represented. 



No calcareous bodies were found in any of the sections. One of the 

 smallest specimens was placed in hydrochloric acid, but no evidence of 

 the presence of calcareous i)articles was educed. 



Structure of the hodij. — There is a rather abrupt transition between 

 the head and the body. When transverse sections made through the 

 anterior part of the body immediately back of tlie head are compared 

 witli those of the head several differences may be observed. The 

 cuticle is thicker and more sharply defined. In the center of the 

 section the connective fibers assume a jiarallelism, especially in a 

 direction from margin to margin, which is in marked contrast with 

 the irregular course which they ])ursue in the head. The open spaces, 

 which indicate the situation of longitudinal vessels, are collected 

 towards the ])eriphery. The protoi)lasmic giannles in the center soon 

 begin to collect into clusters, whicli lepresent the beginnings of the 

 testes. 



In a small specimen about 5 mm. long, which was cut into transverse 

 sections, it was not until the middle, or a little back of the middle, 

 counting from the head, that the longitudinal muscles began to collect 

 into bundles to form a somewhat discontinuous layer surrounding the 

 inner space. 



