202 Introduction to Animal Morphology. 



three pairs of these "segmental" organs develop^ 

 separate from the primitive streak, on the hinder half 

 of the abdomen. These never open inwards, and 

 vanish when the permanent segmental organs appear. 

 Sometimes the wide, pouch-like forms open internally 

 into plexiform canals, formed of an aggregate of large 

 cells with branching spaces between them. These 

 are not sexual in function. 



Leeches are mostly hermaphrodite, but not self-im- 

 pregnating. The sex-organs resemble those of Trema- 

 todes, as a symmetrical pairofeachkindexists, opening 

 ventrally. The testes are either two simple pouches 

 (Pontobdella), or else a row of glomeruli* (Fig. 27, A, /) 

 on each side, whose efferent ducts unite in two lateral 

 longitudinal vasa deferentia, dilating near their end 

 into seminal vesicles, whose ducts unite ; an accessory 

 gland (sometimes acinose, Clepsine) secretes an 

 albuminous matter, which unites the spermatozoa into 

 clumps (spermaphores). The end of the duct may be 

 protrusible as a penis, or there may be a special penis 

 sometimes armed with recurved hooks (Branchio- 

 bdella). The male organs open in front of the female. 

 In Branchiobdella the sixth segment wall secretes the 

 semen, which escapes by two canals to a glandular 

 pouch at the base of the penis. There are two 

 ovaries and oviducts, often with an albumen gland at 

 their confluence into a dilated sac (uterus). The eggs 

 are enclosed in a smooth or rough capsule or cocoon, 

 surrounded by fluid albumen, within which the embryo 

 begins to creep, developing a pharynx ; then a primi- 

 tive streak forms, on which the ventral ner\'e cord 



* Five pair, Ichthyobdella, Branchellion ; 6, Piscicola ; 8, Hsemopsis ; 

 9, Hirudo; I2, Aulacostoma. 



