864 AUSTRALIAN HIKUDINEA, II.. 



ment of characters widely divergent in connection with genera 

 in other respects closely allied. This renders it almost impossible, 

 with our present knowledge, to point to any one genus as being 

 more primitive than another. 



In the case of the Glossijohoniidm, we have the triannulate 

 somite, which undoubtedly is more primitive than that existing 

 in forms with a greater number of annuli, but from this one 

 cannot judge that the family is any older than another. 



The development of a proboscis in the Rhynchobdellidce is to 

 be regarded as a character evolved after the separation of that 

 group from the Arhynchobdellidce. The primitive ancestor of 

 these two groups was not provided with such a proboscis, as can 

 be readily seen from a study of the embryology of the Hirudinea. 



In this connection it is rather interesting to note that the 

 Rhynchobdellids are all aquatic forms, and it seems very likely 

 and probable that these forms have, in their proboscidean nature 

 considered in conjunction with their habitat, some explanation 

 of the origin of the group. 



In the genus which I have described in this paper, I noted the 

 presence of a peculiar proboscis-like outgrowth from the oeso- 

 phagus or from the base of the pharynx. One would at first 

 sight readily regard this structure as a rudimentary proboscis, 

 but snch is apparently not the case. The proboscis of the Rhyn- 

 chobdellids is formed by an involution of the anterior extremity 

 of the body, the pharyngeal sac-wall representing the modified 

 epidermis of the involuted portion. A transverse section of the 

 proboscis of these forms is markedly comparable with the anterior 

 extremity of the body of an Arhynchobdellid. An examination 

 of the structure of the proboscis-like organ which I have 

 described in the new form shows that, in this connection, it is 

 not comparable with the proboscis of Rhynchobdellids. I men- 

 tioned above that very possibly some relationship existed between 

 the presence of a proboscis in the Rhynchobdellids and their 

 aqueous habitat. Now it seems to me that it is quite probable 

 that the same cause may have stimulated the evolution of the 



