HIRUDINEA. 525 



are striking features. The segmentation is unequal. The mouth is formed 

 early, and through it, after the formation of the pharynx and rudiments of the 

 alimentary canal, the albumen contained in the cocoon is taken into the intestine 

 of the growing embryo by means of swallowing movements of the pharynx. 



In the Gnathobdellidae it appears that a considerable part of the la*^a is cast 

 off in the attainment of the adult stage, e.g. the ectoderm, muscles, pharynx, 

 and provisional kidneys. The development* resembles in many respects that of 

 the Oligochaetes. This particularly applies to the origin and fate. of the meso- 

 blastic bands. They arise from two pole-mesoblasts and become segmented into 

 somites, each of which soon acquires a cavity the coelom. The somites spread 

 ventrally, and unite with one another across the middle line. In this ventral 

 part the walls between successive somites break down, and a continuous tube is 

 formed the ventral sinus while in the lateral parts they give rise to the septa, 

 which in an incomplete manner persists throughout life. In this way arise the 

 ventral sinus and the lateral sinuses, while partial obliteration of the connection 

 between these (intermediate sinus of Clepsine) and diverticula of them give rise 

 to the complicated sinuses of the adult. 



The ncphridia arise from a single large cell of the somite-wall, which pro- 

 liferates and produces the characteristic cord, which becomes the nephridium 

 the strings of botryoidal tissue appear to arise in a very similar manner from 

 large cells of the somite wall which produce strings of cells by proliferation. 

 In both cases the cell-cords become hollow, the lumen communicating with the 

 coelom. The botryoidal sinus in Nephelis, which contains the nephridial funnel, 

 is directly derived from the lateral part of the primitive somites, the segmenta- 

 tion of which is retained. If this account of the origin of the botryoidal cords 

 is correct, it would appear that they resemble the nephridia closely in their 

 development, structure, and relations to the coelom, and we must regard them, 

 like the nephridia, as special organs of the coelom. 



The ovaries arise as thickened patches of the coelomic epithelium in parts of 

 the coelom which become constricted off from the rest ; the ducts arise by 

 prolongation of the sac so formed to the skin of the ventral surface. The testes 

 arise on each side from a continuous ridge of cells, which separates from the 

 peritoneum and becomes hollowed out into the testes and vas deferens. 



The terminal parts of the nephridia and generative ducts are developed from 

 ectodermal ingrowths. 



Asexual reproduction is unknown in the group. The leeches live 

 for the most part in water, or temporarily in damp earth. A few of 

 the Gnathobdellidae are true land forms, and have lost the power of 

 swimming. Comparatively few are marine, and they belong to the 

 IchtJiyoMellidae. They move partly by "looping," with the help of 

 their suckers, and partly by swimming with active undulations of the 

 body. Many of them are parasitic on the skin or gills of aquatic 

 animals ; most, however, are only occasionally parasites of the outer 

 skin of warm-blooded animals. They do not feed exclusively on any 

 special genus of animals, and their diet is not always the same in the 

 different periods of their existence. The caeca of the stomach 



* 0. Burger, Zoologische Jahrbucher, 4, 1891, and Z.f. w. Z., 58, 1894. 



