4 8p 



THE ANIMAL PARASITES OF MAN 



F. HIRUDINEA s. DISCOPHORA (Leech). 



THE Hirudinea, which have been appropriately included amongst the Annelida, 

 differ in many respects from the typical members of the group ; their body is long 

 and flat, it lacks the parapodia that are characteristic to all forms of Annelida ; but, 

 on the other hand, possesses a terminal posterior sucker, and in many species there 

 is also an anterior sucker. The mouth is terminal at the anterior end, the anus lies 

 dorsally above the posterior sucker (fig. 3480). The body is 

 segmented, but this is less manifest in the body covering than 

 it is in the arrangement of the internal organs ; the segmenta- 

 tion, nevertheless, is also indicated exteriorly by the appear- 

 ance of the cutaneous sensory organs which correspond to the 

 segments. This shows what the condition of the ganglia in the 

 abdominal ganglion chain has taught us, that the anterior and 

 the most posterior segments are considerably abbreviated a 

 part of the latter taking part in the formation of the suctorial 

 organs. In a great many species the skin is distinctly annu- 

 lated, four or five of such rings, at least in the central region 

 of the body, appearing on one segment of the body. The 

 condition of their body cavity is another peculiarity of the 

 Hirudinea; it is narrowed by the powerful development of the 

 connective tissue and the muscular system into four tubular 

 sinuses, which have the appearance of blood-vessels. There 

 are usually one dorsal and one ventral median trunks, as well 

 as two lateral trunks ; in addition, a particular blood-vessel 

 system exists. 



The skin consists of a very thin cuticle that is cast off from 

 time to time ; it is secreted by the underlying cylindrical epi- 

 thelium, which contains numerous goblet cells. The muscular 

 system is strongly developed ; it consists of long tubular fibres, 

 which run circularly, longitudinally and in the dorso-ventral 

 direction; the muscular system, is subject to a particular 

 expansion in the clinging organs and at the commencement 

 of the intestine. On the whole, the alimentary canal represents 

 a tube running straight from the mouth to the anus, which 

 possesses a number of blind sac-like protuberances at the 

 sides varying according to the species. The most anterior 

 section, the pharynx, in the leeches with maxillae carries three 

 chitinous, semicircular plates furnished with teeth the jaws 

 which serve to tear up the epidermis in order to open the 

 blood-vessels ; in the leeches with rostra a long protractile 

 proboscis rises from the base of the elongated pharynx. 

 Numerous salivary glands, the secretion from which possesses 

 toxic properties, discharge into the pharynx. The oesophagus, 

 which follows the pharynx, and to the exterior of which 

 numerous radiary muscles are fixed, is a suctorial organ in 

 its entire structure. The nutriment in the larger species con- 

 sists of the blood of vertebrate animals, in smaller species and 

 in the young stages the food consists of small invertebrate 

 animals. 



FIG. 3480. The 

 internal organs of 

 the leech. The 

 creature has been 

 opened from the 

 dorsal surface, and 

 part of the intestine 

 has been removed. 

 The testicles, with 

 vas deferens, may be 

 seen between the 

 blind ducts of the 

 intestine ; beyond 

 these on either side 

 the segmental organs. 

 The female genital 

 organs are in front 

 of the most anterior 

 pair of testicles. 

 (After Kennel.) 



