236 Elementary Biology. 



mentary system, the anus. Some distance from the anterior 

 end there occurs a swelling on the body, usually of a 

 yellowish colour, thus contrasting with the dark reddish 

 brown of the general body surface. This swollen portion 

 goes by the name of the clitellum. The clitellum usually 

 occupies the position of the twelve somites after the twenty- 

 eighth. The ventral aspects of the ninth to the fifteenth 

 somites are also slightly tumid, marking the openings of the 

 ducts from the reproductive organs and from certain organs 

 accessory in function to the ovaria and spermaria. These 

 are the chief features which strike the eye in an examination 

 of the external surface of the worm. If, however, a hand 

 lens be employed two minute apertures will be found on the 

 ventral surface of each somite, which mark the openings 

 of the purifactory or renal organs, known in the worm as 

 nephridia. 



We may now suppose the worm- to be laid open by a 

 median longitudinal incision along the dorsal surface. If 

 this incision be carefully made, and if the skin be pinned 

 back, the internal anatomy may be seen with very little 

 supplementary dissection. 



The alimentary system. The alimentary system con- 

 sists of a straight tube running from the mouth to the anus, 

 with certain dilatations at intervals. The dilatations are 

 of great importance as indicating morphological differentia, 

 tions in the alimentary system into parts which have differ- 

 ent functions to perform in the preparation of food. We 

 may note first the cavity into which the food enters, the 

 buccal cavity, which opens immediately into a part of the 

 alimentary tube provided with exceedingly thick walls, known 

 as the pharynx. The anterior part of the pharyngeal wall 

 is composed of muscular tissue, and is connected with the 

 wall of the body by a large number of radiating muscle 

 bands. After the pharynx there follows the oesophagus, a 

 narrow slightly sacculated tube, which opens at its posterior 

 end into a large sacculation or crop. Attached to the wall 



