426 CHARACTERS OF INVERTEBRATE ANIMALS 



is a short feeler or cirrus. Imbedded in each lobe is a bundle of 

 bristles or setcs, of which one is much larger and stronger than 

 the rest, though it only just projects from the surface of the body. 

 These setae are of great use in locomotion, acting 

 as holdfasts by means of which the body gets a 

 purchase on sand or the like. They exhibit all 

 sorts of variations in shape. The head, upon the 

 under side of which is the mouth, consists of a 

 mouth-segment, and a projection in front of the 

 mouth which may conveniently be called the head- 

 lobe. From the upper side of this lobe, near the 

 front, spring two short tentacles, and there are 

 two much larger " palps " which arise from the 

 under side of the same region rather farther back. 



Fig 26 -ASea-centi- ^*-*^ *-*'^^y *^° these last-namcd structures act as 

 pede [,Nenis), diagram- sensory organs, but they also serve as lips to some 

 extent. Upon the upper side of the head-lobe are 

 four simple eyes, looking like black specks. The mouth-segment 

 is provided with four pairs of slender feelers, which are apparently 

 of the same nature as the cirri of the trunk-segments. None 

 of the appendages are converted into jaws, a feature which is 

 so characteristic of Arthropods, though it may be remembered 

 that in the member of that group which comes nearest to the 

 Annelida, i.e. Peripatus, there is only one pair of these structures 

 (see p. 399). Turning now to the last tail segment, which is 

 perforated by the opening of the intestine, we shall find that it 

 is comparatively small and devoid of foot -stumps, though it 

 possesses one pair of long backwardly- directed cirri. 



Internal Structure of Nereis (fig. 265). The body-wall pre- 

 sents very primitive features, reminding one of Peripatus (see 

 p. 401). It consists of the skin, which is practically little more 

 than a thin epidermis covered by a tough cuticle, and two under- 

 lying muscle-layers, of which the outer is composed of fibres which 

 run transversely, while the other consists of four prominent bands 

 in which the fibres have a longitudinal direction. Closely con- 

 nected with the body-wall, though scarcely perhaps forming part 

 of it, is an oblique sheet of muscle on each side, made up of 

 numerous fan-shaped sections, which take origin near the sides 

 of the ventral nerve-cord and spread out in the foot-stumps, to 

 which they are attached and serve to move. 



