CONTRIBUTIONS TO THE ANATOMY OF THE HIRUDINEA. 493 



the extent to which they are developed, the amount of their 

 branching, and, as I find, the histological character of their 

 epithelium. In Aulostoma the alimentary epithelium is 

 ciliated. This has not hitherto been observed, and is very 

 interesting when we consider the difference in the nature of 

 the food of this genus (earth-worms, smaller individuals of its 

 own genus) and the food of Hirudo, &c. The regions are: 

 1, pharynx ; 2, oesophagus and proventriculus ; 3, digestive 

 stomach ; 4, intestine ; and 5, rectum. 



In Hirudo by far the largest portion is the oesophagus and 

 proventriculus with its ten pairs of csecal diverticula j this is 

 followed by the digestive stomach, the very reduced portion 

 with the so-called rectal cseca, and then the intestine and 

 rectum. This very small stomach is related to the very slow 

 absorption which takes place in Hirudo. In other genera 

 the corresponding region is much more fully developed. There 

 is probably no absorption taking place in the portion with the 

 diverticula, the epithelium in this region possibly secreting a 

 preservative or digestive juice. In transverse sections the cells 

 may be seen giving off droplets of a mucous-like substance 

 which have sunk into the blood-mass (fig. 66, tn). 



ix. Phylogenetic Conclusions. 



The place of the leeches in classification is a question which 

 has at all times attracted a considerable amount of attention. 

 I do not propose to give here any history of the older views 

 upon the subject. 



Two main views are held at the present day. Either the 

 Hirudinea are Platyhelminthes specially related to the Trema- 

 todes and Triclada, or this resemblance is a superficial one, and 

 they are more closely related to the Chsetopoda. 



The former of these views has been insisted upon by P. J. 

 Van Beneden^ and Leuckart,^ who hold that they are allied to 



> Van Beneden and Hesse, ' Recherches sur les Bdelloides ou Hirudinees et 

 les Trematodes Marins,' Brussels, 1863. 

 ' ' Die Mensch. Parasiten,' 1S65. 



