DEVELOPMENT 



275 



These larvae are at first very different from the adult animal, 

 and the necessary changes to be passed through are more or less 

 great according to the species. 

 It is not our intention to 

 describe these changes in de- 

 tail. 1 The larva increases in 

 size, the permanent chaetae 

 make their appearance in regu- 

 lar order, and the body exhibits q/ 

 segmentation, the new seg- i 

 ments always appearing just 

 in front of the anal segment. 

 The internal organs gradually 

 dev.elop, and the prostomial and 

 parapodial appendages grow 

 out in their turn. In the 

 Sabelliformia the multifila- 

 mentous " gills " arise by the 

 continued branching: of an at 



Fig 



. 145. A, Trochosphere of NepMhys. 

 x 65. a, Anus ; b, apical plate (brain) ; 

 c, apical tuft of cilia ; c', girdle of cilia ; 

 i, intestine ; m, mouth ; st, stomach. 

 B, Larva of Spio, with three segments, 

 eight days old. x 100. c, Preoral 

 girdle of cilia ; c', preanal girdle ; ch, 

 long provisional chaetae ; pr, prostomium 

 with eyes. (From Claparede and Metsch- 

 nikoff.) 



first simple process (the palp) 

 arising from the latero-ventral surface of each side of the preoral 

 lobe. 2 These gradually encroach dorsally and ventrally till the 

 prostomium is more or less encircled ; meanwhile the peristomium 

 grows forwards so as to conceal the prostomium, which no longer 

 increases at the same rate as does the rest of the body. 



Although most worms appear to discharge their ova directly 

 into the sea and take no further care of them, some make 

 provision for their offspring either by laying the eggs in a jelly, 

 which will serve as food for the young larvae Aricia, Ophelia, 

 Protala, Phyllodoce or by attaching them to their body. In 

 certain Polynoids the eggs are attached by means of a secretion 

 to the back, under the elytra, where they undergo development 

 up to a certain stage. In Exogone and some other Syllids 

 they are attached to the ventral cirri, or in Grubea limhata, 

 all over the back. In the female Autolytus (Sacconereis) a 

 ventrally - placed brood sac is formed by the hardening of a 



1 For an account of the anatomy and development of a Trochos] there, see Hat- 

 schek, on Enpomatus, in Arbeit. Zool. Inst. JFicn, vi. 1885. Also ^leyer, Jit. 

 Zool. Stat. Ncapcl, viii. 1888, p. 462 ; and for Polynoid larva see Hacker, Zool. 

 Jaltrb. Abth. Anat. viii. 1895, p. 245. 



2 See Meyer (ref. on p. 261). 



