ANNULATA. 259 



swells, then becomes milky white, from the formation of a film, into 

 which the animal, having attached itself by its anal sucker, forces, 

 after some effort, the whole contents of the uterus. This being done, 

 the leech elongates the anterior part of its body, and, thus loosening 

 the enveloping membrane, withdraws its head as from a collar. It 

 sometimes bends back its head, and, drawing the collar forwards, 

 gently aids in its removal. The process usually occupies about twenty 

 minutes. The cocoon is at first very elastic, and has no determinate 

 figure. After the leech has attached to it some adjoining substance, 

 it fashions it with its mouth into an oval form. The points of the 

 cocoon from which the leech withdrew its head are weaker than the 

 rest, and from these the young escape. 



Mr. Brightwell, who has carefully observed this common species of 

 our freshwater pools, states: — "On the 2nd of June, H. vulgaris 

 deposited one capsule containing ova ; on the 5th, another ; on the 

 10th, another; and on the 15th, two more ; each of them containing 

 from seven to ten eggs. On the 22nd, the young appeared in the 

 capsule deposited on the 2nd ; and on the 13th of July they emerged 

 from the capsule, and in six weeks were fully developed, and left the 

 capsule. Examining the young of tliis species with a power of about 

 sixty linear, we detected a Cypris and four specimens of a common 

 rotiferous animalcule in its stomach, one of the Rotifera being still 

 alive."* 



Both Weber t and Filippi J have figured ova of the leech-tribe in 

 certain stages of cleavage ; but the primary and subsequent steps of 

 this important process in anellidous ova were first noticed by Kolliker § 

 in a small Nereis (^Exogone Orstedii), with the impregnated ova con- 

 tained in marsupial sacculi arranged in two rows along the ventral 

 side of the body ; each sacculus held a single ovum, and was sus- 

 pended, a pair to each segment from the tenth to the twenty-third 

 inclusive, by a short stem to the ventral integument. These sacculi 

 are probably formed by albuminous mucus, exuded and hardened by 

 sea-water. Kolliker figures the single central germ-cell ; the double 

 germ-cell, followed by the total fission of the yolk with the secondary 

 germ-cell in each ; these cells again divided, and the consequent 

 quadrifid division of the yolk ; folloAved by successive subdivisions, 

 as in the Ascaris. 



In the e^g of CIepsine\\, after the disappearance of the germinal 

 vesicle a whitisli discoid mass of molecular corpuscles appears at one 

 end of the yolk. Assuming this to mark the pole, the first line of 

 cleavage is a meridional one, Avhich bisects the disk unequally : 



* CXCVII. t CXCV. t CXCVIII. § CC. II ccV. 



s 2 



