28o 



THE INVERTEBRATA 



it the chain of sexual individuals which one by one detach and lead 

 a short independent existence. In some species of Trypanosyllis 

 (Fig. 194 C) the zone of proliferation is in the form of a cushion of 

 tissue on the ventral surface of the last 

 two segments and this produces not a 

 linear series of buds but successive trans- 

 verse rows, amounting to more than a 

 hundred — the fully formed sexual indi- 

 vidual possesses a head but no vestige of 

 an alimentary canal. The extraordinary 

 branching form, Syllis ramosa (Fig. 

 194 D), shows remarkable capacity for 

 heteromorphic growth in the production 

 of sterile side branches from the stock 

 and reproductive buds. 



In the syllid there is usually no noto- 

 podium during asexual life but during 

 the maturation of the gonads the para- 

 podium is reconstructed, a notopodium 

 being formed from which spring bundles 

 of long capillary swimming chaetae, 

 while a corresponding development 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 digested by leucocytes Fig. 195. A Heteronereis. )^ho- 

 before the new muscles are formed. The tograph of specimen stained 

 parapodium of the sexual form, the in borax carmine and mounted 

 Heteronereis /is produced into memhr^n- in Canada balsam Notice the 

 - .,, J ^ . , r enormously developed eyes, 



ous frills and contains a new type ot j^^^ peristomial cirri, anterior 

 oar-shaped chaeta (Fig. 184 D, F). The unmodified trunk region, pos- 

 eyes become immensely larger and the terior modified region with 

 animal itself very sensitive to light. The parapodia sloping backwards 



TT • J • r ^ ui 4-u and darker appearance owmg 



Heteronereis does in fact resemble those ^^ ^^^^^^^^ of gonads. 



members of the Phyllodocidae and 

 Alciopidae which have become perma- 

 nently pelagic. The increase 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 (Odontosyllis) where the meeting of 



