2OO Introduction to Animal Morphology. 



branches of the first ventral ganglion unite to form an 

 upper, lobate, pharyngeal ganglion, made of several 

 little clumps of nerve cells. No branches arise from 

 the inter-ganglionic cords, but the twigs from the 

 ganglia are numerous and symmetrical. An indistinct 

 third central cord may exist in some parts of the nerve 

 axis. The neurilemma contains muscle-cells, and is 

 double, the layers being separated by pigment cells. 

 A single nerve lies beneath the intestine, send- 

 ing branches to the caeca. It has ganglion cells 

 along its whole course. Curious senses-organs exist 

 in the form of " cup-like organs," most abundant on 

 the head, and hinder, but not hindmost, rings. Each 

 of these is a little depression covered by clear cells, 

 and having a nerve twig terminating in the centre 

 (Ilirudo, Haemopsis, Nephelis, &c). These resemble 

 the organs of Semper in Gephyn-a, or, more remotely, 

 the lateral line organs of fiMirs. Some of these have 

 pigment flecks, and are then considered eyes, but they 

 are possibly only general sense-organs ; of these there 

 may be two (Clepsine), four (Piscicola), eight or ten 

 (Hirudo). In the common leech they are in two series 

 on the i -3rd and 5~8th rings. In it, also, a row of 

 white spots on the horizon of the eyes marks the 

 division into zonites. Limbs are absent, or as flat 

 lateral appendages (Branchellion, Branchiobdella). 



The mouth is in or below the anterior sucker, lead- 

 ing into a muscular pharynx, sometimes protrusible as 

 a proboscis, which has then special retractor muscles, 

 and no teeth. Others have 2-3 plates of calcified 

 chitin (jaws), with serrated edges (teeth), or unarmed. 

 Brown, oral, salivary cells may line the pharynx, 

 opening outwards like the glands of Trematodes. The 



