232 



IN VERTEBRA TE MORPHOL OGT. 



ce 



n 



FIG. 108. DIAGRAM OF 

 THE EXCRETORY RE- 

 PRODUCTIVE AND NER- 

 VOUS SYSTEMS OF Hirudo 

 (after BOURNE). 



ce = cerebral ganglion. 

 ep = epididymus. 

 gl oviducul gland. 

 lv = lateral blood-vessel. 

 n = nephridia. 

 ov = ovary. 

 pe = penis. 

 te = testis. 



masses. Six of these 



Macrobdella) it is thrown into three 

 longitudinal muscular ridges whose 

 edges may become converted into 

 chitiu, thus forming teeth. Salivary 

 glands open into the pharynx in some 

 forms. The large stomach into which 

 the pharynx opens behind gives off a 

 number of lateral pouches (eleven 

 pairs in Hirudo, seven in Clepsine), 

 sometimes branched and increasing in 

 size from before backwards, the most 

 posterior pair being usually quite long 

 and directed backwards parallel to the 

 straight narrow intestine which opens 

 to the exterior on the dorsal surface of 

 the body, just anterior to the posterior 

 sucker. Occasionally only the poste- 

 rior pair of pouches is present, and in 

 a few forms they are entirely wanting. 

 The nervous system (Fig. 108) is 

 constructed on the typical Annelid 

 plan. It consists of a circunioesopha- 

 geal ring and a ventral nerve-cord 

 composed of fibres which have their 

 origin in ganglion-cells grouped to- 

 gether at definite intervals into gan- 

 gliouic masses. Several of these gan- 

 ' gliouic masses correspond to single 

 segments, but at the anterior and 

 posterior extremities a considerable 

 amount of fusion of the inetameric 

 groups of ganglia has occurred. In 

 Clepsine plana the portion of the ner- 

 vous system which lies above the 

 oesophagus consists of a transverse 

 band of fibres passing lateral!}' into 

 the circumoasophageal commissures 

 and of a number of ganglionic 

 latter lie in front of the baud of fibres 



