192 THE ANATOMY OF INVERTEBRATED ANIMALS. 



and in relation with nerves which terminate in fine filaments, 

 have been discovered by Leydig in several of the Hirudinea. 1 



The elongated spindle-shaped muscle-cells of the body are 

 abundant, and are disposed in a superficial circular and deep 

 longitudinal layer, while dorso-ventral bands pass from the 

 dorsal to the opposite body-wall. 



Malacobdella and Histriobdella are dioecious, but the other 

 Hirudinea are hermaphrodite. The male organs consist of 

 numerous testicular sacs, situated on each side of the body, 

 and connected by a vas deferens, which usually opens into 

 a sac, terminating in an eversible penis. The spermatozoa 

 are often inclosed in a case or spermatopliore. The female 

 organs, much smaller than the male, consist of ovaries, with 

 oviducts opening into a vagina. The vaginal orifice is behind 

 that of the penis. In the Leech the eggs are inclosed in a 

 sort of cocoon, formed by a viscid secretion of the integu- 

 ment. 



The observations of Rathke and Leuckart on the develop- 

 ment of JVephelis, Clepsine, and Hirudo show that, after the 

 division of the vitellus into a few equal-sized large blasto- 

 meres, small blastomeres are separated from the large ones (as 

 in the Ctenophora and JPolycelis), and the rapidly-multiply- 

 ing small blastomeres form an investment to the slowly-divid- 

 ing large ones. This investment is the epiblast, and becomes 

 the ectoderm, while the included larger blastomeres are event- 

 ually converted into the cells of the endoderm. At one end 

 of the body the oral aperture appears, in some cases (e. g., 

 JVephelis) surrounded by a raised lip, as in the embryo Pla- 

 narian ; and the embryo passes into the Gastrula stage. The 

 body now elongates, and, on the ventral face, the mesoblast 

 makes its appearance as a layer of cells, sometimes divided 

 into two longitudinal bands, separated by a median interval. 

 Three pairs of segmental organs, w r hich have only a tempo- 

 rary existenoe and have been regarded as primordial kidneys, 

 are developed at the posterior end of the body. The meso- 

 blast next becomes divided transversely into the number of 

 somites of which the body is eventually composed, the divis- 

 ion first making its appearance on the ventral face of the 

 body. A pair of ganglia, probably derived from the epiblast, 

 is developed in each segment. 



Thus, in the Leeches, the segmentation of the bodr is the 

 result of the segmentation of the mesoblast, which becomes 



1 " Archiv fur Anatomie und Physiologie," 1861. 



