POLYCHAETA 



247 



I 



ment of new muscles takes place. Even greater is the change in 

 the parapodia of the maturing nereids. 

 The muscles of the asexual period 

 break down and the fragments are di- 

 gested by leucocytes before the new 

 muscles are formed. The parapodium of 

 the sexual form, the Heteronerets, is pro- 

 duced into membranous frills and con- 

 tains a new type of oar-shaped chaeta 

 (Fig. 173 D, F). The eyes become im- 

 mensely larger and the animal itself very 

 sensitive to light. The Heteronereis does 

 in fact resemble those members of the 

 Phyllodocidae and Alciopidae which have 

 become permanently pelagic. The in- 

 crease in the surface of the parapodia 

 may be useful in swimming and floating : 

 it has without doubt some connection 

 with the increased gas exchange associated 

 with an active life. 



It is easy to see in the swarming habit 

 an adaptation for securing fertilization 

 of the greatest possible number of eggs. 

 There are remarkable cases in the syllids 

 where the meeting of the sexes is facili- 

 tated by the exchange of light signals, 



and in the nereids the discharge of sperm in borax carmine and mounted 



111 1 ^ u ^ L ^u • in Canada balsam. Notice the 



may only be brought about by the m- ^^^^^^^^^^ developed eyes, 



fluence of a secretion from the swarmmg i^^g peristomial cirri, anterior 



female. Discharge of the gametes is unmodified trunk region, pos- 



nearly always followed by the death of terior modified region with 



the sexual individual. parapodia sloping backwards 



rr^i r M- 1 • • . and darker appearance owmg 



The fertilized egg gives rise to an un- ^^ presence of gonads, 

 segmented larva, the trochosphere, which 

 is described in the next section. 



Fig. 184. A Heteronereis . Pho- 

 tograph of specimen stained 



Development of the Polychaeta 



The cleavage of the egg in the Polychaeta and the Archiannelida, 

 the polyclad Turbellaria, the Nemertea and the MoUusca follows 

 almost exactly the same plan. Division occurs rhythmically, affecting 

 the whole or greater part of the blastomeres at the same time. The 

 first two divisions are equal, producing four cells (Fig. 185,2) lying 

 in the same plane, which are called A,B,C,D\ each cell in its further 



