x PHYLUM ANNULATA 409 



internal in which they run longitudinally. The latter is not a 

 continuous layer, but consists of four bundles of fibres, two dorso- 

 lateral (dors. long. mus.~) and two ventro-lateral (vent. long. mus.). 



Nereis has a well-developed system of vessels filled with blood 

 of a bright red colour. A main dorsal vessel (Figs. 321 and 322, 

 dors, ve&j) runs from one end of the body to the other above the 

 alimentary canal, and is visible in places through the body- wall in 

 the living animal. It, as well as the majority of the vessels, un- 

 dergoes contractions which are of a peristaltic character waves of 

 contraction passing along the wall of the vessel so as to cause the 

 movement of the contained blood. These peristaltic contractions 

 are more powerful in the case of the dorsal vessel than in that of 

 any of the others, and run with great regularity from behind for- 

 wards, so as to drive a current of blood in that direction. The 

 contractions are brought about partly by a series of muscular fibres 

 which are arranged in rings round the wall of the vessel at short 

 intervals ; but the wall of the vessel is itself contractile. 



Along the middle of the ventral surface below the alimentary 

 canal runs another large longitudinal vessel, the ventral vessel (vent, 

 ves.), in which the current of blood takes a direction from before 

 backwards. Connecting the dorsal and ventral vessels, there are in 

 each segment two pairs of loop-like transverse vessels which give 

 off branches to the parapodia, the alimentary canal, and neighbour- 

 ing parts. Some of these branches communicate with plexuses 

 of fine vessels in the interior of the lobes of the parapodia and in 

 the integument of the dorsal surface, and with dilatations or sinuses 

 situated in the bases of the parapodia. A delicate longitudinal 

 neural vessel accompanies the nerve-cord. 



Nereis is devoid of any branchial ; but there can be little doubt 

 that the lobes of the feet with their rich blood-supply, and the 

 areas of integument occupied by plexuses of blood-vessels, subserve 

 the function of respiration. 



There is a well-developed nervous system (Fig. 323) which is 

 bilateral and metameric in its arrangement, like the other systems 

 of organs. Situated in the prostomium is a large bilobed mass 

 of nerve-matter containing numerous nerve-cells, the cerebral 

 ganglion or brain (c). This gives off tentacular nerves to the tentacles 

 and palpi, and two pairs of short thick optic nerves to' the eyes. 

 Behind, two thick nerve-strands, the cesophageal connectives (d), curve 

 round the mouth in the peristomium to meet on the ventral 

 aspect behind the mouth and below the pharynx. The cesopha- 

 geal connectives, with the cerebral ganglion, thus form a ring 

 around the anterior part of the enteric canal. Running back- 

 wards from the point of union of the cesophageal connectives, 

 along the entire length of the body of the worm, on the ventral 

 aspect, is a thick cord of nerve-matter, the ventral nerve-cord (h) 

 In each segment this cord presents a little dilatation from which 



