510 ZOOLOGY SECT. 



the medicinal leech, as much blood as takes it a year to digest. 

 The allied species Hirudo sanguisuga has been found in the nasal 

 passages of man, producing serious results, and being, to all 

 intents and purposes, an internal parasite. The same is the case 

 with the horse-leech, Hcemopsis vorax, taken in, when young, by 

 horses and cattle while drinking. It attaches itself to the pharynx 

 and may even descend the trachea. Others are permanent ecto- 

 parasites : for instance, Branchellion occurs on the outer surface 

 of the Skate, Electric Hay, and other Fishes, entire families of this 

 leech, including individuals of all sizes, being sometimes found 

 crowded together on a small area of skin, which is distinctly 

 marked by their powerful posterior suckers. Other fish-parasites 

 are Pontobdella, on Rays, and Piscicola, on fresh-water Fish. 

 Aulostoma, to which, as well as to Hsemopsis, the name Horse- 

 leech is applied, is carnivorous, feeding on snails and other 

 Molluscs ; so also are Glossiphonia (Clepsine), Herpobdella (Nephelis), 

 and the gigantic Macrobdella. The last-named genus and some 

 others are of subterranean habits, living in moist earth. The 

 Land-leeches (Hcemadipsa) live in the forests of many parts of 

 the world, and in spite of their small size, which does not exceed 

 30 mm. in length and 5 mm. in diameter, are much dreaded for 

 the persistent attacks they make on men and cattle. 



Many genera are very widely distributed : for instance, the 

 Land-leeches (Hcemadipsa) occur in India, Ceylon, the East Indies, 

 Japan, Australia, and South America, a distribution which seems 

 to indicate that the group is one of great antiquity. 



GENERAL REMARKS ON THE ANNULATA. 



A special feature of the Annulata, as distinguished from the 

 phlya previously dealt with, is metameric segmentation. In some 

 of the Platyhelminthes, as we have seen, and in Gordius to a less 

 extent, there obtains a condition to which the term pseudo- 

 metamerism is applied. In such cases there is a serial repetition of 

 certain of the organs gonads, diverticula of the intestine, nerve- 

 commissures, &c. in such a way as to produce a jointed appearance, 

 though the body is not divided into definite segments. An appear- 

 ance resembling segmentation is produced also in certain Rhabdo- 

 cceles that multiply by budding, chains of zooids remaining connected 

 together for a time. In the strobila of the Cestodes we recognise a 

 condition which might be described as combining pseudo-meta- 

 merism with the formation of a chain of zooids. The condition of 

 true metamerism, as we observe it in the Annulata, is capable of 

 being deduced from a condition of pseudo-metamerism as it occurs 

 in Gunda (p. 251), the pseudo-metameres becoming converted into 

 true metameres by the development of inter-segmental constric- 

 tions and the completion of internal partitions. If we suppose that 



