Leeches 23 



responds with that of maximum temperature of the water in August. 

 While the excursions of the leeches into the open waters are con- 

 cerned primarily with the quest for food they serve also to extend 

 and alter their distribution. They are strong and rapid swimmers, 

 progressing with easy and graceful dorso-ventral serpentine move- 

 ments during which the body may be turned either vertically on edge 

 or flatwise. 



Every feature of their organization shows that these animals are 

 beautifully adapted to a habit of occasional and temporary parasitism 

 and a sanquinary diet. The acute chemical and mechanical senses, 

 the powerful organs for rapid and varied locomotion, the soft and 

 sinuous body, the concealing coloration, the strongly developed 

 organs of attachment, the finely fashioned instruments for painlessly 

 cutting the skin and capillaries, the haemolysin and powerful pump- 

 ing apparatus for facilitating the rapid flow of blood, and the great 

 storage capacity and ability to subsist without food to tide over 

 periods of scarcity are among those that appeal especially to students 

 of adaptation. 



When biting the leech usually attaches itself firmly to its host by 

 the caudal sucker, the anterior end of the body meanwhile exploring 

 for a suitable place to bite, preference being given to a cut or abraded 

 spot, or, in the absence of such, to a tender vascular area of the skin. 

 To this the oral sucker is then applied intimately and fixed firmly, 

 and the jaws are pushed forcibly against the surface. By a rotary 

 back and forth motion toward and from the center of the radially 

 arranged jaws effected by two sets of oblique muscles, the numerous 

 fine teeth cut three narrow deep slits together having a characteristi- 

 cally trifid pattern. The salivary secretion, containing the haemolysin 

 called hirudin, pours over the jaws directly into the wound and mix- 

 ing with the blood keeps it fluid and facilitates its flow. By the action 

 of the bulbous pharynx the blood is pumped from the wound into the 

 gastric or crop caeca which may become filled in about fifteen or 

 twenty minutes. If the stomach of a feeding leech be cut into from 

 the exterior the blood will flow from the opening in pulsations. The 

 quantity of blood taken by leeches of the size commonly found in 

 Carr Pond is from six to fifteen grammes but may vary between 

 much wider limits depending upon the size of the leech and the nature 

 of the host. 



After completing a meal the leech drops oflf and being then 

 negatively phototropic (that is, tending to move away from the source 

 of light) it seeks the darkness under stones or logs, in crevices, or in 

 burrows in the mud. Ordinarily it does not feed again for several 

 weeks or even months, but is less abstemious than the European 

 medicinal leech, which is said habitually to allow five or six months 

 to elapse between meals. A leech may live indefinitely without food. 

 Two years and more has been recorded for the European leech and I 

 have kept Macrohdella decora alive without food for from seven to 

 fifteen months. Under such circumstances they shrink greatly in 

 size. For example, in the experiment covering fifteen months the 

 average weight of five leeches fell from 3.09 grammes to 0.79 



