ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 517 



of tlie sense-organs. The terminal somites are of the highest importance 

 for specific diagnosis, and the annular composition, which offers so mnchi 

 of theoretical interest, cannot be deciphered without the use of these 

 organs. 



But the importance of the segmental character of the sense-organs 

 is not to be measured by its usefulness in systematic determinations ; 

 nowhere is a chapter in the evolution of sense-organs so perfectly Tpre- 

 served as among the Hirudinea. These segmental organs appear to be 

 identical with the lateral-line organs of Vertebrates, and it is suggested 

 that they have formed the starting point for the organs of special sense 

 in the higher animals, not excepting even the eyes of Vertebrates. Mr. 

 Whitman thinks that if we take what are now incontestable facts in the 

 phylogeny of annelid and arthropod sense-organs, and add to them the 

 evidence in favour of the common derivation of the vertebrate organs of 

 special sense, we shall not much longer be able to concede to the 

 visual organs of Veitebrates the position of isolation they have so long 

 held. In the study of this question we must remember that (1) verte- 

 brate sense-organs must be assumed to be derived from invertebrate 

 sense-organs, and the history of the latter must furnish clues to the 

 genesis of the former ; (2) in the development of sjiecial senses visual 

 cells have made the widest departure from the primitive tactile cells ; 

 (3) the medullary plate of the vertebrate is undoubtedly an enormous 

 extension of the ancestral invertebrate plate ; (4) sense-organs lying 

 originally outside the neural plate have probably, in consequence of this 

 extension of width, been brought within the medullary area ; (5) the 

 ancestral segmental sense-organs were not limited to a single j^air of 

 lateral lines, but there were several paired lines arranged symmetrically 

 on the dorso-lat'^ral and ventro-lateral surfaces. 



A careful analysis of the annular composition of the body of Clepsine 

 has enabled the author to find just twenty-six somites in front of the 

 caudal sucker. Adding seven for the sucker, we have thirty-three, so 

 that the number of somites determined by the external rings agrees 

 precisely with the number of ganglia in the ventral chain. 



The nervous system of BranchelUopsis is exceptionally interesting 

 from the possession of veritable spinal ganglia ; they are lodged in the 

 anterior (sensory) of the two sj^inal nerves of each somite at a short 

 distance from the ventral cord. A j^air of colossal nerve-cells are found 

 between every two consecutive ganglia in the ventral cord of this leech. 

 They contain axial cells which undoubtedly corresjjond to the neuro- 

 chord cells of other Annelids and probably to the colossal nerve-fibres 

 of Amphioxus, Miiller's fibres in Petromyzon, and Manthner's fibres in 

 Teleosteans. 



In Clepsine clielijdrse the spinal nerves issue as three distinct roots, 

 the anterior of which unites with the middle to form one nerve. The 

 agreement in form and structure between Piscicolaria and the Japanese 

 Bi-anchelliopsis is remarkable, for it is much closer than that between 

 the fresh-water Piscicola of Europe and marine leeches. 



All the Hirudinea may be derived from a form in which the somite 

 consists of three rings; the author promises to explain in an early paper 

 how these rings may become 4, 5, 6, or 12. Copulation in Clepsine 

 is never direct, that is, by union of sexual pores ; as in Neplielis and 

 Peripatus, the spermatozoa are transmitted in sj^ermatophores which are 

 planted on any part of the exterior, preferably on the back. The 



