52 ESSENTIALS OF ZOOLOGY 



in or about the brain of the sheep. An allied form passes 

 between the rabbit and the dog. In the rabbit the coenurus 

 is lodged usually near the armpit. 



The tapeworm chosen for special study is Taenia serrata, 

 which occurs in the adult condition in the dog and is derived 

 from the Cysticercus pisiformis so common in the peritoneal 

 cavity of wild and domestic rabbits. 



The adult tapeworm is fastened to the wall of the intestine 

 of the dog by the head or scolex. The scolex is provided with 

 a ring of about thirty-six hooks, alternately large and small, 

 situated on a prominence called the rostellum, below which 

 are four suckers, all about equally distant from one another in 

 a transverse row. This region is succeeded by a short unseg- 

 mented neck, only slightly narrower than the scolex. The rest 

 of the body is about a yard in length and is segmented, each 

 segment being called a proglottis. The proglottides are flat 

 expansions of the body and gradually increase in width. At 

 first that is, near the neck the segments are short, at a 

 distance of about a foot from the scolex they are almost as 

 broad as long, and the length gradually increases in the follow- 

 ing segments. With increase in length the proglottides 

 present lateral projections which bear the genital pore in each 

 case (fig. 23). 



The structure of this tapeworm as a whole is simple. The 

 ectoderm forms an external covering and the cells project 

 below into the muscle layer ; it secretes a cuticle externally. 

 Below the ectoderm there are longitudinal muscles and cir- 

 cular muscles, and the rest of the space internally is occupied 

 by a mesenchyme in which certain other organs are embedded. 

 There is no mouth nor alimentary canal. Two longitudinal 

 nerves, one on each side of the worm, run from end to end 

 and merge in the scolex in a ring. The excretory system 

 consists of longitudinal canals which receive branches all along 

 their length, branches which begin in flame cells or soleno- 

 cytes. It is a protonephridial system, and it opens terminally 

 on the last segment by a single pore which is renewed as the 

 segments are cast off. The food is imbibed through the 

 cuticle which protects the worm from the digestive juices of 

 the host ; it is already digested and is circulated in the 



