ANNULATA. 261 



proportionally, especially the testes, more minute than the mucous 

 loops and round mucous sacs, whose development keeps pace with 

 that of tiie general integument. The circulation can be discerned 

 before the pigment-cells are formed in the skin ; and the large lateral 

 veins are early conspicuous. The superficial cilia disappear ; and the 

 embryo, which hitherto, by its vibratile surface and single opening of 

 the digestive cavity, has resembled a planaria, now acquires an anus. 

 The yolk-mass is progressively pushed by the growth of the integu- 

 ment to the dorsal and anal parts of the body, the last remnant 

 appearing as a conical papilla above the anal sucker. The internal 

 yolk-mass is divided by the growing septa or constrictions of the 

 stomach, the intervening dilatations of which are afterwards developed 

 into sacculi, and of these the last pair is already the largest ; the 

 remainder of the yolk is inclosed by the intestine, whose outlet 

 surrounds the external projecting part. The alimentary canal con- 

 tracts and expands on the contained yolk -material. This course of 

 development extends over six weeks. Weber traced it from the 6th 

 of July to near the end of August. At this time, the young move 

 about in the cocoon, which may contain from five to fifteen ; the 

 comparative paucity of the j'oung, even when a vigorous leech suc- 

 cessively forms and fills as many as six cocoons, agreeing with the 

 small size and number of the ovaria. They escape by perforating 

 one end of the cocoon, and are subtransparent, but show as many 

 rings as the parent. But little growth takes place up to the month 

 of December : the young get sustenance by sucking the mucus of the 

 parents, but will grow, if isolated, by assimilating tlie remnant of the 

 yolk-mass in the stomach. The period of acquiring the full growth 

 and power of propagation depends much on the opportunities the 

 young leech may have of sucking the blood of other animals. John- 

 son believes that five years elapse before the medicinal leech is 

 full-grown, and speaks of one which he kept alive twenty years. Do 

 Blainville found twp-year-old leeches only three inches long. Audouin 

 could not succeed in keeping them beyond from eight to twelve years, 

 when they were allowed to propagate. The Naides and ArenicolcB, 

 according to the author of the *' Report,"* are annuals. " They are 

 born during the latter months of one summer, and survive the winter, 

 attain to the maturity of growth, reproduce the species, and die by 

 the spontaneous subdivision of the body into fragments on the arrival 

 of the same season of the succeeding year." 



Koch "I" found a specimen of Eunice, probably E. satiguinea, with 

 developed embryos in most of the joints ; some of the hindmost seg- 



* CXC. t CCVII. 



s 3 



