segmented or metameric condition of the body. In the Annulata 

 of which the beach-worm, Nereis, the earthworm, Lumbricus, and 

 the leech, Hirudo, may be taken as examples, the segmentation is 

 homonomous, each metamere being similar to the others, with 

 the exception of an imperfectly developed prostomium, enclosing 

 the brain and overlapping the mouth, a peristomium, surrounding 

 the mouth, and a terminal metamere containing the anal aperture. 

 Each metamere bears primitively a pair of flattened appendages, the 

 parapodia; there is a coelomic cavity lined by a layer of peri- 

 toneal cells, a nervous system, the principal portion of which is 

 in the form of a ventrally placed double nerve-cord, and there are 

 segmental organs, nephridia, a pair in each segment, which 

 provide a communication between the coelomic cavity and the 

 outside of the body. 



Most annulates are free-living, but the division of the Hirudinea 

 is made up of forms which are partly parasitic. The medicinal 

 leech, Hirudo medicinalis, formerly used for removing blood from 

 the body of patients, shows externally a subsidiary segmentation 

 in which five external rings correspond to one true or internal 

 segment. The body is dorso-ventrally flattened. There is an 

 anterior and terminal sucker, lodging the aperture of the mouth, 

 and containing three cutting or lacerating jaws, a feature of the 

 Gnathobdellida to which the medicinal leech belongs. A pos- 

 terior sucker, also terminal but on the ventral side, is for attach- 

 ment only. The anal aperture is placed above it. The digestive 

 tube is expanded into a crop or reservoir for the reception of blood 

 upon which the animal feeds. 



The Hirudinea are for the most part parasites of lower aquatic 

 vertebrates. In some (Rhynchobdellida) the mouth is borne on 

 a protrusible proboscis, ordinarily found retracted into its sheath. 

 Limnatis nilotica, an African form of 8 to 10 cm. in length, some- 

 times lives as a parasite in the human nose and pharynx, the young 

 gaining access to the host through drinking water. Haemadipsa, 

 a tropical genus, includes a number of species significant as human 

 parasites, one of which, H. ceylonica, inhabiting Ceylon, lives in 

 moist earth from which it ascends trees, afterwards dropping on 

 the bodies of human beings or animals passing by. 



ARTHROPODA. 



The phylum Arthropoda, containing many parasites and 

 carriers of parasites, has in many respects the fundamental organiza- 

 tion of the annulate phylum from which its members are in all 

 probability derived. The segmentation is, however, heteron- 



14 



