160 



ANNELIDA 



[CH. 



of separate sexes, and the eggs develop into a larva which swims 

 in the sea and gradually changes and grows up into a worm. This 

 group includes a very great variety of forms, almost all of which 

 are marine. With few exceptions they form burrows for themselves, 

 which most of them occasionally desert in order to seek prey and 

 to discharge the reproductive cells. Some however never leave the 

 burrows, which in this case often take the form of tubes composed 

 of a secretion of the ectoderm. 



1 



Order III. Hirudinea. 



Besides the Oligochaeta and Poly- 

 chaeta the order Hirudinea, the 

 members of which are popularly known 

 as leeches, is included amongst the 

 Chaetopoda. They were for some time 

 regarded as a distinct order of Annelida, 

 since the great majority of species 

 possess no chaetae and have other 

 peculiarities; but the recent discovery 

 of species possessing chaetae, and the 

 close resemblance between the develop- 

 ment of all Hirudinea and that of 

 Oligochaeta, renders it evident that 

 they are true Chaetopoda and that the 

 absence of chaetae is a secondary 

 characteristic. 



There is little doubt that the 

 Hirudinea are closely al- 

 lied to the Oligochaeta; 

 indeed there are certain 

 families which it is not easy to assign 

 definitely to either group ; but the 

 more typical forms are easily distin- 

 guished. Externally leeches may be 

 recognised by the possession of a sucker 

 at each end of the body, the anterior 

 one being formed by the mouth, whilst 

 the posterior one is a special organ. 

 By alternately attaching and releasing 

 these suckers and bending the body the animal crawls along. 



Leeches. 

 External 

 features. 



FIG. 67. Hirudo medicinalis, 

 about life size. 



1. Mouth. 2. Posterior 



sucker. 3. Sensory papillae 

 on the anterior annulus of 

 each segment. The remain- 

 ing four annuli which make 

 up each true segment are in- 

 dicated by the markings on 

 the dorsal surface. 



