196 MANUAL OF ZOOLOGY. 



The sexes in the Erraniia are in different individuals, and 

 reproduction is usually sexual, though in some cases gemina- 

 tion is known to occur. The process of geminatibn is carried 

 on by a single segment, and so long as it continues, the bud- 

 ding individual remains sexually immature, though the young 

 thus produced develop generative organs. Thus, there is in 

 these cases a kind of alternation of generations, or rather an 

 alternation of generation and gemmation ; the oviparous indi- 

 viduals producing eggs from which the gemmiparous indivi- 

 duals are bom ; these, in their turn, but by a non-sexual pro- 

 cess, producing the oviparous individuals. 



The embryo usually appears, on its liberation from the ovum, 

 as a free-swimming, ciliated body, possessing a mouth, intestine, 

 and anus. The cilia are primarily diffused, but become aggre- 

 gated so as to form a single median belt, or two bands, one 

 about each extremity. The head, with its feelers and eye- 



Fig. 59-:—" Errant " Amielide Nereis, showing the " head " with its appendages, 

 and the setigerous parapodia. 



specks, appears at one extremity, whilst the segments of the 

 body begin to be formed at the other. Each segment is devel- 

 oped in four parts, the two principal ones forming half-rings, 

 united by shorter side-pieces, from which the setigerous foot- 

 tubercles are developed. The ciliated band or' bands finally 

 disappears, and new rings are rapidly added by intercalation 

 between the head and the segments already formed. 



Amongst the best known of the Errantia is the common 

 Lob-worm (Arcnicola piscatorum, fig. 60, C), which is used by 

 fishermen for bait. The Lob-worm lives ip deep canals which 

 it hollows out in the sand of the sea-shore, literally eating its 

 way as it proceeds, and passing the sand through the aliment- 

 ary canal, so as to extract from it any nutriment which it may 

 contain. It possesses a large head, without eyes or jaws, and 

 with a short proboscis. There are thirteen pairs of branchise, 

 placed on each side in the middle of the body. 



Tn th<5 A^ereida, or " Sea-centipedes," the body is greatlv 



