ANNULOSA: ANNELIDA. 167 



double row of imbricated plates, which are called ' elytra,' or 

 ' squamae,' and respiration is effected by the periodical eleva- 

 tion and depression of these plates, whereby water is alter- 

 nately admitted into, and expelled from, a space beneath them. 



Fig. 47. < Errant' Annelide. Nereis, showing the 'head' with its appendages, 

 and the setigerous parapodia. 



This space is separated by a membrane from the perivisceral 

 cavity below. 



The nervous system in the Errantia has its typical form, 

 consisting of a double gangliated ventral cord, two ganglia of 

 which are appropriated to each segment. The prse-oesopha- 

 geal, or cerebral, ganglia are of large size, and send filaments 

 to the ocelli and feelers. 



The sexes in the Ermntia are in different individuals, and 

 reproduction is usually sexual, though in some cases gemma- 

 tion is known to occur. The process of gemmation 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 born ; these, in their turn, but by a non- sexual process, 

 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- 

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

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

 loped 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 

 disappear, and new rings are rapidly added by intercalation 

 between the head and the segments already formed. 



