The Earthworm. 23 



sperniathecal pores. Finally, there is the mouth in front, 

 already referred to as lying beneath the prostomium, and the 

 anus posteriorly, entirely surrounded by the last segment of 

 the body. 



Such are the principal external characters of the earthworm. 

 It will be observed that the setee, the dorsal pores, and the 

 nephridiopores show the same plain segmentation as the 

 divisions of the body to which they accurately correspond. 



When an earthworm is opened by a median incision along 

 the back, and the flaps of skin turned back, the entire anatomy 

 is revealed. 



The Body-wall. — This flap that has just been turned back 

 is built up of three layers. Outside there is an epidermis, 

 within that two muscular layers, an outer circular and an inner 

 longitudinal, shown in the accompanying figure (Fig. 10). 



The alimentary t7-act is seen passing in a perfectly straight 

 line from end to end of the body, and from it a series of 

 delicate transverse septa reach the body-wall, forming a means 

 of suspension of the digestive tube. These septa divide the 

 body into a set of chambers, through which the digestive tube 

 passes, but which it does not entirely fill. There is left a 

 considerable space, above, below, and at the sides of the tube. 

 This cavity, or series of cavities, is known as the body cavity, 

 or cmlom. The septa, roughly, not always quite accurately, 

 correspond at their insertion on to the body-wall to the furrows 

 which separate the segments externally. The interior of the 

 worm's body is therefore segmented like the exterior, and the 

 internal segmentation corresponds to the external. The 

 chambers of the coelom, which are divided from each other by 

 the successive septa, are not, however, completely separated. 

 On the ventral side the septa are defective, being cut away 

 along a curved line, so that there is an actual communication 

 between the whole series of coelomic compartments, and the 

 fluid which is contained therein can pass from end to end of 

 the body. Lying in each compartment of the coelom is a 

 pair of delicate coiled tubes, the excretory organs, or nephridia. 

 The term " segmental organ " was originally applied to 

 these glandular tubes in order to express their segmental 



