214 CLASS VII. 



colour shining through the skin, which is white in male individuals 

 and reddish in female. Other differences of external appearance 

 in the two sexes are not known, unless we except an observation of 

 OERSTED, which however is not altogether free from doubt, accord- 

 ing to which in a new genus very nearly allied to Syllis, which he 

 names Exogone, the male individuals are distinguished by longer 

 hairs, as they are in the genus Nais 1 . There still remain the 

 genera of the Hirudinea and Lumbricmi, in which STEENSTRUP in- 

 deed adopts separation of sex under similar external form of the 

 parts : but this requires confirmation after accurate investigation, for 

 it is in conflict with earlier observations, whilst by later it is in part 

 contradicted 2 . On the whole, no common type can be assigned for 

 the genital organs : for the most part, there are some pairs of vesicles 

 (ovaria, testes) in the fore part of the body. In some Annulata setigera, 

 apertures at the base of the foot-swellings have been seen, through 

 which passes seed or eggs : but in many of them such an outlet is still 

 unknown. For the most part, external genital organs are deficient : 

 neither does copulation occur, except in Lumbricini and Hirudinea. 



The development of the egg has been investigated only in a 

 few species. Here also that remarkable cleaving and successive 

 division of the yelk has been observed, which KiJSCONi and YON 

 BAER first detected in the eggs of frogs and of fishes. The deve- 

 lopment of the embryo begins on the abdominal surface, and the 

 yelk lies on the dorsal surface, as in Crustaceans and Insects : two 

 abdominal streaks are observed at the commencement of develop- 

 ment, which recall the dorsal plates of vertebrate animals 3 . 



The most recent times have made us acquainted with some 

 remarkable metamorphoses in the course of the development of 

 ringed- worms. LOVEN found the first stage in a worm of the 

 family of the Nereids (probably a species of Phyllodoce) to resemble 



1 EBICHSON'S Archivf. Naturgesch. 1845, i. s. 20 23. 



2 See F. MUELLEE on the Hermaphroditism of the Hirudinea, in the German 

 translation of STEENSTRUP'S work cited above (p. 135) Untersuchungen ueber das 

 VorJcommcn des Hermaphroditismus in der Natur. Greifswald, 1846, s. no 114. 



3 Most of the observations refer to Hirudinea. Such are the following works : 

 E. H. WEBER, Ueb. die EntwicTcelung des medicin. Blutegels, MECKEL'S Archiv. 1828, 

 s. 366 418, Taf. x. xi.; R. WAGNER, Bruchstucke aus der Entwickelung des gemeinen 

 Bluteycls, Hirudo vulgaris L., Nephelis tesselata Sav. OKEN'S Isis, 1832, s. 398 408, 

 Taf. iv.; A. E. GRUBE, Untersuchungen ueber die Entwickelung der Clepsinen. Mit 3 

 Kupfert. Konigsberg, 1844. We may expect several observations on marine annulata 

 from QUATREFAGES. See Ann. des Sc. nat. 3101116 Serie, Zoologie i. p. 21. 



