ANNELIDA. 81 



duced into the vagina of another individual. In this process two 

 individuals mutually fertilize each other, their ventral surfaces 

 being applied and their anterior ends turned in opposite directions. 



7. Muscular System. The body-wall, within the skin, consists 

 of three muscle-layers, an external, a middle, and an internal. 

 The thin external layer is composed of fibres taking a circular 

 direction, and within this comes the thin middle layer in which 

 the fibres are oblique, while the very much thicker internal layer 

 is made up of numerous bundles of longitudinal fibres. There are 

 also numerous dorso- ventral muscle-bands, the ends of which 

 spread out in a fan-like way within the epidermis. The suckers 

 are made up of radial and circular muscle-fibres. 



The muscular tissue is composed of elongated spindle-shaped 

 muscle-cells, each of which consists of a longitudinally striated 

 external (cortical) part and a granular nucleated internal (medul- 

 lary) part. The former is specially contractile, the latter proto- 

 plasmic. 



Locomotion is chiefly effected by a looping movement or by 

 swimming. In the first method the posterior sucker is attached 

 to some object and the body stretched forwards by contraction 

 of the circular muscle-layer. Then the anterior sucker is attached 

 and the body is dragged forwards by contraction of the longi- 

 tudinal layer. Swimming is a much more complex affair ; in it 

 the body is moved in a wave-like fashion. 



8. The nervous system consists of a very narrow nerve-ring 

 round the pharynx, a ganglionated ventral cord, a sympathetic 

 system, and nerves connected with these. 



The nerve-ring is thickened above into cerebral ganglia, which 

 supply the jaws, sense-organs of the head, and other anterior 

 structures. The sides of the ring are connectives which unite 

 these ganglia with a pair of infra-wsophageal ganglia, from which 

 five pairs of nerves are given off. The ventral cord is made up 

 of two closely united longitudinal halves, which enlarge to form 

 a pair of ganglia in the first annulus of each segment. Of the 

 23 pairs thus constituted the first and largest are the infra- 

 cesophageal ganglia. Then follow 21 much smaller pairs, from 

 each of which two dorsal and two ventral nerves are given off to 

 the corresponding segment, and the cord is terminated by the 

 somewhat larger 23rd pair, which supply the posterior sucker 

 and give origin to a number of nerves. 



6 



