ON THE ANAT(^MY OF HISTRIOBDELLA HOMARI. 317 



tial details with that of Histriobdella as far as can be 

 judged from Haswell's somewhat brief description. There is 

 the same reduction of the tract in the generative region, this 

 being much greater in the fenmle than in the male, and its 

 expansion into a more oi- less large hind-gut in the caudal 

 region. 



As compared with Dinophilus there is a greater difference. 

 Yet with the exception of the peculiar mid-gut portion of the 

 tract, which is a development due to the peculiar condition 

 produced by the presence of a special generative segment, 

 there is considerable resemblance between Histriobdella 

 ajid Dinophilus, and in many of the finer histological 

 details there is a very close resemblance. In the first place, 

 the appearance of the cells of the stomach, each composed of 

 a single layer of ciliated cells, the yellow vacuolated appear- 

 ance of their protoplasm, and the basal arrangement of the 

 nuclei, ai'e the same in the two. The terminal dorsal position 

 of the anus and the configuration of the oesophagus and 

 pharynx are remarkably the same in both. 



According to Nelson (25) there is a feeble strand of muscle- 

 fibi-es that act in Dinophilus as sphincter ani, as in 

 Histriobdella. Throughout the stomach region there is a 

 lack of muscular strands, and the stomach is not supported 

 by mesenteries, but is closely applied to the dorsal wall, as 

 in Histriobdella. The blastocoelic surface of the stomach, 

 as in Histriobdella, is covered with a fine cuticle. 



The jaw apparatus of Histriobdella is very similar to 

 that of Stratiodrilus. Haswell has given an extensive 

 description of this, so that I need only briefly consider it. 

 As in Stratiodrilus, it consists of two portions — the upper 

 and the lower. The upper consists of a median rod (fig. 36), 

 which Haswell has called the fulcrum. This is slender, 

 round, and slightly curved ; it articulates by means of a 

 number of basal pieces with a series of jointed arms, each 

 terminating- in a curved tooth (text-fig. 5). It lies in the 

 median plane dorsal to the two blades of the lower jaws, being 

 set at a different angle to these. Its length is somewhat less 



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