ANNELIDA 



297 



"ganglion" (Fig. 205 B) is composed of four single ganglia fused 

 together and the posterior "ganglion" of seven. Between them lie 

 twenty-one free ganglia, and the number of segments is estimated by 

 summation of all the ganglia. The number of annuli to a segment 

 varies in different forms. 



Fig. 205. Anterior part of nervous system in A, Lumhricus. After Borradaile. 

 B, Hirudo. After Leydig. The brain in both consists of a single dorsal pair 

 of gangHa belonging to the prostomium. In Lutnbricus the subpharyngeal 

 ganglion (sbp.) and lower part of the circumpharyngeal commissures give 

 off nerves to segments i (peri.) peristomium, 2, 3 and so belong to three 

 segments. In Hirudo the subpharyngeal mass consists of four (or five) pairs 

 of ganglia fused together, e. eyes ; M.c. mouth cavity ; 7. jaws ; pr. prostomium ; 

 ph. pharynx (with network of visceral nerves) ; so. sense organs. / 



The alimentary canal is highly characteristic and consists of the 

 following parts, (i) A muscular pharynx with unicellular salivary 

 glands. In the Gnathobdellidae, which includes Hirudo, there are 

 three chitinous plates or jaws. In the Rhynchobdellidae (Fig. 206), 

 there is a protrusible ^ro^o^m surrounded by 2i proboscis sheath. (2) A 

 short oesophagus follows, leading into (3) the mid gut (crop) which is 



