TYPE ANNELIDA. 237 



Development of the Hirudinea. — The Gnathobdellidse deposit their eggs 

 in chitinous cocoons, as do the Oligochaeta, and the development is of the 

 foetal type, iu contradistinction to the larval, the ova containing as a rule a 

 considerable amouni of yolk. The mode of oviposition of the majority of 

 the Rhynchobdellidsb is unknown ; but in the genus Clepsine the eggs are 

 fastened to the ventral surface of the body of the parent, where they un- 

 dergo development. This resembles closely the development of Luiribricus, 

 allowing for the greater amount of yolk which is usually present. The 

 same precocious segregatiou of mesoderm, nervous system, and nephridia 

 in special budding cells, ihe niesoblasts, neuroblasts, and nephroblasts, is 

 likewise found, aud in later stages the mesoblast is distinctly segmented 

 and coelomic cavities are present, which later become to a great extent 

 obliterated. 



The Affinities of the Hwudinea. — It is exceedingly probable that the 

 ancestors of the Hirudinea were to be found in the Oligochaeta, the two 

 groups having not a few structural features in common. The embryologi- 

 cal peculiarities found in the two groups are strikingly similar ; and fur- 

 thermore the aquatic or terrestrial habits are not a little suggestive, for 

 although some leeches are marine, nevertheless the majority are aquatic 

 and a few terrestrial. The complete disappearance of parapodia may be 

 considered a further development of the tendency towards their oblitera- 

 tion in the Oligochaeta, where only the setae are present, these even having 

 disappeared in the Hirudinea in consequence of the development of the 

 suckers and a new mode of locomotion. The suggestive arrangement of 

 the sense-papillsB of the Oligochste Slavina has already been mentioned . 



It must not be forgotten, however, that the difierenoes between the two 

 groups are many and important. Such are, for instance, the disappearance 

 of the original coelomic spaces, the communication of the blood vascular 

 system with sinuses, and the occurrence of special ducts for the reproduc- 

 tive organs. These differences have, however, equal or even greater im- 

 portance when the attempt is made to trace the Hirudinea directly to the 

 Polychaeta, and it seems more satisfactory at present to refer them back to 

 the Oligochaeta. 



III. Class Uephyrea. 



The Gephyreans constitute a group of niarine worms whicli 

 differ from the Clisetopoda principally in the more or less 

 complete absence of metamerization. All trace of it is ab- 

 sent upon the outside of the body; for although the thick 

 cuticle may be marked by distinct rings, these bear no relation 

 to the internal parts and are, as in the Nematoda, due simply 

 to the thickness of the cuticle. All traces of parapodia are 

 lacking in many forms, while in others they are represented 



