ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 341 



It is pointed out that G. segmentata presents many features of 

 striking resemblance to certain Hirudinea, and especially the Bhjncoh- 

 dellidce ; the pharynx, like that of the Triclades, is contained in a 

 special cavity ; the intestine has always a number of paired diverticula, 

 the number of which is constant for a given species. The two last 

 are always longer than the others, and often have, on their outer 

 side, secondary outgrowths. These may be compared to the lateral 

 and posterior branches of the intestine of the Triclades. The 

 terminal intestine, the posterior dorsal anus, and the large sucker are 

 to be regarded as formations special to the Hirudinea. 



There is likewise a considerable resemblance as regards the 

 excretory system, but the collecting organ of the Hirudinea is, again, 

 a new formation ; in the adult leech there is no connection, as we 

 know, between the excretory system and the enteric diverticula, but 

 in the embryos of Clepsine there is evidence that this system is 

 developed from the epithelium of these diverticula. Striking resem- 

 blances are also to be seen in the generative system. The ventral 

 ganglionic chain of the Hirudinea does not appear to be so very 

 different, if we suppose that it is comparable to the two longitudinal 

 nerve-trunks of Gunda connected at segmental intervals by simple 

 commissures. 



The musculature of the Hirudinea is mesenchymatous ; the uni- 

 cellular muscular fibres consist of an axial substance with a nucleus 

 and a contractile sheath, just as in Gunda there is a dorsal muscula- 

 ture consisting of an external layer of transverse muscles, and an 

 internal one of longitudinal fibres. In addition, there are dorso- 

 ventral muscles which cannot be distinguished from the muscular 

 dissepiments of Gunda, and, just as in that form, there is no enteric 

 muscular layer. The body-cavity of the Hirudinea is not an entero- 

 coole, but a schizocoele, formed by the vascular and lymphatic systems 

 which are in communication with one another, and are developed, as 

 Prof. Lankester has shown, by the liquefaction of the parenchymatous 

 cells of the mesenchyma. Were the diverticula of the intestine to 

 be detached from it, we should have a true enteroccele, which would 

 then give rise to the epithelial musculature of the wall of the body 

 and of the intestine, the excretory organs would thus acquire their 

 primitive relations to the diverticula, and would serve, at the same 

 time, for the evacuation of the generative products. It is probably 

 along some such lines as these that the Oligochata and Annelids 

 have been developed from a Leech-like form. 



In connection with this subject Dr. C. Chun * points out that, 

 though thei'e are several points in common, there are also some 

 important differences in the development of the Ctenophora and 

 marine Planaria. In both there are four small and four large 

 cleavage-spheres, and the gastrula is formed by epiboly. While, 

 however, the rapidly multiplying small cells of the Ctenophora 

 represent the rudiments of the ectoderm and mesoderm, in the 

 Planaria there arise four primitive mesodermal cells, which alone 

 form the mesoderm. He is not certain that the resemblances point 

 * Biol. Ceufralbl., ii. (1882) pp. .5-16. 



