IV POLYCHAETA 



147 



ance^ just in front of the sexual region, a constriction of the body which 

 gradually deepens so that the whole of this portion of the body becomes 

 completely separated off (Fig; 70, II). This latter retains its vitality 

 for some time, wriggling about and distributing the gametes. Another 

 species of Syllis {S. hyalina) behaves in the same way as S. hamaia but 

 in this case the separated-off portion after a few days develops a definite 

 head region so that it now forms a complete sexual individual. In 

 various species of Autolytus the sexual individual develops its head 

 before it separates off (Fig. 70, III) and further the process is repeated 

 — new sexual individuals being produced in succession, each after its 

 predecessor has separated off. In certain exceptional cases sexual 

 individual No. 2 (Fig. 70, IV, z) develops before No. i separates off and 

 this leads up to the condition met with in old specimens oiA. edwardsii 

 where as many as six new sexual individuals may be recognizable 

 before the first-formed one has become detached (Fig. 70, V). In a 

 closely alUed genus, Myrianida, chains of as many as thirty sexual 

 individuals have been observed — the youngest and least developed one 

 in front — attached to the hinder end of the original individual. This 

 latter is of course asexual, developing no gametes, though it may be 

 said to reproduce asexually by the production of the new sexual individuals 

 at its hinder end. 



These reproductive peculiarities which find their chmax in Myrianida 

 are of special interest from their parallelism with phenomena character- 

 istic of one of the great groups of parasitic worms dealt with in the 

 next chapter — the Tapeworms or Cestoda. 



While Nereis exemplifies satisfactorily a typical member of the 

 Polychaeta there occur within the group many variations in details of 

 structure. A general idea of the kind of variations met with is got by 

 examining a few common marine genera. One of the features which 

 is particularly apt to depart from the normal is the dorsal or notopodial 

 cirrus. In Cirratulus, common amongst mud and sand under stones 

 near low- water mark, the dorsal cirri are long and threadlike, twisting 

 actively about in the live animal and functioning as gills. In Eunice — 

 found in burrows in sand near low-water — the dorsal cirrus also functions 

 as a gill but here it develops side branches so as to have a feathery 

 appearance. In Polynoe — one of the commonest genera of marine worms 

 — the back is covered by overlapping plates or elytra, each of which is 

 really the greatly expanded tip of a notopodial cirrus which has grown 

 out in mushroom fashion all round (Fig. 71, A, d.c). In Aphrodiia — 

 the "Sea-mouse" to give it its somewhat absurd popular name — an 



