CHAETOPODA. 605 



The spermatozoa of Oligochaeta are contained in spermatophores, as is 

 the case also in Spio Mecznikowianus, the sole instance among Polychaeta 1 . 

 The ova are invested by a vitelline membrane, and are generally coloured. 

 They are impregnated externally to the body, or in transit (?), except 

 probably in a few viviparous forms (Marpkysa sangttinea, Syllis vivipara, a 

 Cirrhatulus) ; and in Oligochaeta are laid in a cocoon secreted by the 

 clitellum. They are carried under the elytra in Polynoe cirrata ; attached 

 to a dorsal or ventral cirrus in the Exogoneae (of Langerhans) among 

 Syllidae, and in the female Autolytus cornutiis { Sac c oner eis] they are 

 carried and develope in a ventral sac. In Spirorbis P agenstecheri ( = Sp. 

 spirillum of Pagenstecher), and some other Sabellids 2 the ova develope in a 

 cavity of the operculum ; in Spirorbis spirillum (of Gould), and Manayunkia 

 they are contained within the tube ; in Terebella, Dasyckone, &c. they are 

 attached externally to the tube ; and in Pkyllodoce, Aricia, Ophelia^ they 

 are laid in a mass of jelly-like mucus. 



Segmentation is with few exceptions, e. g. Serpula> unequal. The 

 Oligochaeta have a direct development. A ventral ciliated furrow appears 

 to be common, and in Lnmbricus trapezoides its cilia extend round the 

 mouth also. The young embryo of the same worm divides into two, and 

 each half proceeds to develope normally. The Polychaeta have a more or 

 less pronounced metamorphosis with the exception of the Syllidian Exo- 

 goneae. The larva is a Trochosphere with a prae-oral lobe of variable size, 

 a small body somewhat pointed posteriorly. The body of the adult is 

 formed by the growth and segmentation of this pointed region. The 

 Trochosphere is ciliated, and the differing modes of arrangement of the 

 cilia have given rise to various descriptive terms, to which a classificatory 

 value has sometimes been given. One and the same larva may have at 

 different times differing arrangements, and the larvae of allied species may 

 be completely unlike 3 . The terms alluded to are as follows: Atrochae 

 with the cilia scattered uniformly at first, though disappearing in places 

 subsequently, but never so as to form bands ; Mono- or Cephalo-trochae with 

 a prae-oral band ; Telotrochae with both prae-oral and peri-anal bands ; 



female somites develope at tlie sexual period a saccule contiguous to the external aperture, which 

 becomes filled with sperm. The male and female Capitella capitata possess a pair of ciliated 

 saccules between the seventh and eighth somites, opening both externally and internally. They are 

 filled in both sexes by sperm ; ova appear to issue by them in the female, and there are special setae 

 close to their external apertures in the male. The homology of these saccules is doubtful. Their 

 external aperture is close to the intersegmental furrow, and they appear relatively late (Eisig, Mitth. 

 Zool. Stat. Naples, i. 1879, P- JI 4)- The structures termed ' elliptical or seminal pouches,' occurring 

 in the sexual somites of Syllideae, are perhaps nephridial vesicles (cf. Viguier, A. Z. Expt. (2), ii. 

 1884, p. 86), unless they are, as Robin appears to have found them to be in Pionosyllis, a collection of 

 glands (Bull. Soc. Philomath. Paris (7), vii). 



1 Claparede and Mecznikow, Z. W. Z. xix. 1869, p. 171. 



2 Sp. laevis, Salmadna aedificatrix, Psygmobranchus caecus, and Pileolaria sp. ? 



3 E. g. Terebdla nebulosa is lelotrochous, T. chonchilega nototrochous. 



