CONTRIBUTIONS TO THE ANATOMY OP THE HIRUDINEA. 497 



of another system ; while certain systems, the nervous for 

 instance, present a most marked uniformity throughout the 

 group and that in no primitive condition. It is not unusual 

 to find a nervous system in a primitive condition in animals 

 otherwise highly developed, e. g. Peripatus, but it is unusual 

 to find an organ so primitive as the nephridial tubules of 

 Pontobdella appear to be, in connection with such a well- 

 developed nervous system. This becomes more noteworthy 

 when we find that in an allied genus Clepsine presenting a 

 precisely similar nervous system, a highly developed condition 

 of the nephridia obtains. We must regard the Hirudinea as 

 presenting a number of isolated genera belonging to two 

 groups, a large number of intermediate forms having been 

 lost. These genera appear to have had an ancestor presenting 

 as high a development of each system of organs as is found in 

 any single genus of living Hirudinea. 



2. — Anatomical Structure in relation to Allied 

 Genera. 



There can be no doubt that we must look for structural 

 resemblances to the Leeches in the Platyhelminthes on the one 

 hand, and in the Chaetopods on the other: Gunda segmen- 

 tata among the Platyhelminthes, and Branchiobdella 

 among the Chsetopoda, presenting special resemblances to the 

 group. But I consider it quite impossible to show that the 

 Leeches are either more highly developed Triclada or that they 

 are degenerate Chaetopoda ; in other words, the genetic relations 

 are indirect, and not direct, as has been sometimes stated. 



Although in most respects the Leeches present a much more 

 highly developed condition than is found in any Platyhel- 

 minth, they do exhibit certain well-marked affinities with the 

 latter, in both positive and negative characters. The presence 

 of median generative pores in similar positions in all the genera 

 of Leeches, in connection with a hermaphrodite condition, is a 

 very marked Platyhelminth character. 



The suckers also resemble those of Trematodes, but, as 



