228 ZOOLOGY. 



The leech is hermaphroditic, while in certain allied forms 

 (Histriobdella, etc.) the sexes are distinct. 



The eggs of leeches are laid in sacs, or, as in Clepsine, the 

 fish-leech, are covered with a transparent fluid substance, 

 which hardens and envelops the eggs. The Clepsine re- 

 mains over the eggs to protect them until they hatch ; and 

 the young, after exclusion, fix themselves to the under side 

 of the parent, and are thus borne about until they are fully 

 developed and able to provide for themselves (Whitman*). 

 The changes in the egg of Clepsine, after fertilization, are 

 very complicated, and have been described by Whitman. 

 The egg subdivides into a bilateral mass of cells called a 

 blastula ;\ agastrula, and finally a "neurula" stage, charac- 

 terized by the formation of a "primitive band" like that of 

 insect embryos. Soon after attaining the latter stage the 

 embryo hatches and attaches itself to its parent. The mouth 

 is then formed, the nervous system^ arises from the ecto- 

 derm, the segments are indicated, the original number being 

 thirty-three, the segmental organs develop from the meso- 

 derm at about the time of hatching, and about six days after 

 the neurula leaves the egg the eyes become visible. The 

 innermost germ-layer (endoderm) does not arise until eight 

 days after hatching, and by this time the digestive tract is 

 perfected ; the muscular walls of the alimentary canal being 

 derived from the mesoderm. 



* The Embryology of Clepsine. By C. O. Whitman. Quarterly 

 Journal of Microscopical Science. July, 1878. 



f Whitman states that a morula, as defined by Haeckel, does not 

 occur in the developmental history of Clepsine, and he states that when 

 the cleavage process of the egg has been carefully studied it has been 

 found to result in the production of a bilateral germ or blastula, and 

 not a morula. " ' A solid sphere of indifferent cells' is, to say the 

 least, a very improbable form, so improbable that its existence may be 

 held questionable until established by positive evidence. The doubt 

 is all the more justifiable, as more careful investigation has in many 

 cases already shown that the so-called mulberry stage is not a morula, 

 but a blastula or even a gastrula." (Whitman.) 



$ There is originally a pair of ganglia in each of the thirty-three 

 segments ; four of these are consolidated into the suboesophageal gan- 

 glia, eight in the ganglia of the disk, and four in the terminal ganglia 

 of the body. (Whitman.) 



