524 



ANNELIDA. 



FIG. 426. a, cocoon ; ft, female organs 

 of Hirudo medicinalis (after 

 Leuckart). 



the spermatozoa appear to make their way through the skin and 

 tissues to the ovarian tubes, where it is probable that fertilization 



occurs.* This process of cutaneous 

 injection of spermatozoa is found in 

 other groups, amongst which may be 

 mentioned Turbellaria and Peripatus. 



The eggs are usually laid in cocoons 

 (Fig. 426 a) either on stones or plants 

 (Nephelis, Clepsine), or in damp earth, 

 the animal, if aquatic, leaving the 

 water for the purpose. The cocoon is 

 formed by the clitellar glands. When 

 the eggs are about to be laid the 

 leech attaches itself firmly by its 

 ventral sucker, and, twisting itself 

 about, envelops the anterior part of 

 the body with a viscid mass, which 

 covers especially the genital rings like a girdle and gradually 

 hardens to form a firm membrane. A number of small eggs 'and 

 a considerable quantity of albuminous matter in the case of the 

 Gnathobdellidae, then pass out, and the animal withdraws its 

 anterior end from the barrel-shaped membrane, which now contains 

 eggs, etc., and which, after the animal has left it, becomes in 

 consequence of the narrowing of the external openings a tolerably 

 completely closed cocoon. 



The number of eggs contained in a cocoon varies, but is never 

 large. The eggs of the Gnathobdellidae are small, and the young 

 are hatched early and float as larvae in the albumen which they 

 swallow. In the Rliynchobdellidae the eggs are larger and contain 

 more yolk ; they are hatched at a later stage. Clepsine (see p. 526) 

 attaches its cocoon to stones and broods over it till the young are 

 hatched, which takes place at an early stage ; the young then attach 

 themselves to the ventral side of the mother, and are carried about 

 by her, living on albumen secreted by her. 



The development is characterized by the early specialization of the so-called 

 pole-cells (neuro-nephroblasts, mesoblasts), and by the formation from them of 

 cords of cells with a definite destination. This characteristic is not confined to 

 the embryo, but is found also in the adult, where the rows of cells of which the 

 nephridia and botryoidal tissue are botli formed, not to speak of the egg-strings, 



* Vide Whitman, " Spermatophores as a means of hypodermic impregnation," 

 Journ. Morph.j 4, 1891. 



