ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 841 



cells are not placed directly on the entire tract, but on tlie walls of 

 the surrounding capillaries. The muscular elements of the body are 

 all smooth. 



The author does not find himself able to agree with Leydig in 

 asserting the presence of a closable orifice connecting the lymphatic 

 cavity of L. variegatus with the outer world, and he carries his doubts 

 so far as to suggest a re-examination of the characters of Enchytrceus 

 latus and E. galba. 



The histological processes which obtain in the formation of the 

 various organs in the growing caudal extremity and in regenerating 

 portions of the annelidan body are exactly the same as those which 

 obtain in the development of the embryo. We find, that is, that the 

 mesoderm arises from an ingrowth of cells developed at the point 

 where the ecto- and endoderm pass into one another ; this mesoderm 

 soon forms two mesodermal bands which undergo segmentation before 

 the neural ectodermal thickening ; but no mesodermal elements enter 

 into the formation of the nervous portion of the ventral chain, as they 

 do in Nats (Semper). The " primitive nerve-fibres," " giant-fibres," or 

 " neurochord " are not of a nervous nature, but serve as elastic sup- 

 ports for the body. Inasmuch as tbey arise from the mesoderm and 

 not from the endoderm, they are not homologous with the chorda dorsalis 

 of Vertebrates. The muscle-plates and the other muscular elements 

 are of mesodermal origin, as are also the segmental organs, the " liver- 

 cells," and the blood-vascular system. The setee and the nervous 

 lateral lines arise from the ectoderm. The important dificrences 

 between the account given by Semper of .Nais, and that now given by 

 Biilow affect (1) the formation of the mesoderm, and (2) the origin of 

 the " spinal ganglia." 



The former difference is probably to be explained by Semper's 

 sections having been about four times as thick as' those of the more 

 recent observer ; for this is clearly a matter of considerable impor- 

 tance when we reflect that the invagination of the mesoderm may not 

 take place along more than 1-lOOth mm. The same explanation 

 will hold with regard to the ganglia, as to the exact history of which 

 Semper himself emitted some doubts. 



The author concludes with the generalization that the three layers 

 that can be distinctly recognized in the normal growing caudal end of 

 Annelids are the dynamical equivalents of the embryonic germinal 

 layers, since they give rise to the same organs. The only modifica- 

 tion is to be seen in the mode of origin of the mesoderm, which does 

 not, as in the embryo, arise from the endoderm, but from the point 

 where the inner and outer caudal germ-layers pass into one another. 



Anatomy of the Hirudinea.* — A. G. Bourne has investigated 

 ten genera of Hirudinea. He finds that in Pontohdella the external 

 evidences of metamerism are most complete, and that they have a 

 precise relation to that expressed by the internal organization ; the 

 normal somite here presents four annuli of varying size. The cli- 

 tellum includes two reduced somites, and the nerve-cord exhibits 



* Proc. Eoy. Soc, xxxv. (1883) pp. 350-7. 

 Ser. 2.— Vol. III. 3 L 



