152 ^- G0T6: 



shape of the hooks already described, and the raised part chitinised.'^ 

 It should, however, be observed that the process of absorption has pro- 

 bably played as much part in the formation of the hooks as that of 

 chitinisation. 



The penis in Octocotyle is essentially similar to that in DicUdopliora, 

 but its hooks are more like the atrial spines of Microcotyle. In Onclio- 

 cotijle the penis is nothing else than a mass of elastic connective tissue 

 around the terminal portion of the vas deferens, which is separated 

 from the surrounding mesenchyma by a membrane, and is therefore 

 essentially similar to that of DicUdopliora. The hooks are, however, 

 totally abselit. 



The atrial spines of Microcotyle are, as already described, formed 

 by local, conical elevations and chitinous transformation of the lining 

 membrane of the genital atrium, and are therefore homologous in a 

 broad sense with the chitinous penis and penis hooks of other genei'a 

 as well as with the hooks which are pi'esent in many genera in the 

 posterior sucker or in the posterior part of the body, and with the 

 local chitinous areas of the lining membrane of the suckers (PI. VI, 

 fig. 4; PI. XII, fig. 4). I also believe that the chitinous frame-work 

 of the suckers of many genera is homologous with the structures just 

 mentioned, i. e., that it is a product of the local transformation of 

 the investing membrane of the body. 



The homologue of the penis of Monocotyle and Hexacotyle and 

 therefore of Tristomum can also be traced in Microcotyle. It has already 

 been stated that in some species of this genus, as M.fusiformis (PL V, 

 fig. 1), M. caudata (fig. 3), and M. sebastis (PI. VI, fig. 1), the vas 

 deferens opens at the top of a conical papilla projecting into the genital 

 atrium. This papilla is, in my opinion, the homologue of the penis. 



1). Here I have for convenience used -oi false expression. In poinfc of fact the process 

 01 raising and that of chitinisation probably proceeed simultaneously. 



