522 ZOOLOGY SECT. 



but also forms the head the germinal bands not extending into 

 that region. The embryonic enteric cavity (mesenteron) becomes 

 formed by arrangement of the endoderm cells round the three 

 megameres, which break up to form nutrient material or yolk 

 destined to become absorbed in nourishing the embryo. The 

 pharynx is formed by an invagination of the ectoderm which 

 joins the mesenteron. At this stage the embryo leaves the egg, 

 and soon escapes from the cocoon to pass through its later stages 

 attached to the ventral surface of the parent. 



In the Gnathobdellida the young are hatched at an early stage 

 of development, and their megameres contain but little yolk : 

 they are nourished up to the time of leaving the cocoon on the 

 albumen with which the latter is filled. One member of this 

 order, Herpobdella- (Nephelis) is remarkable for undergoing a 

 metamorphosis : the anterior end of the embryo is ciliated, and it 

 possesses a provisional pharynx and several pairs of provisional 

 nephridia. Paired masses of cells, the head-germs, are developed 

 in the head, and from these and the germinal bands the whole 

 body of the adult is produced, the greater part of the larval body 

 being cast off. This process closely resembles the develop- 

 ment of the pilidium larva of certain Nemerteans (p. 295). 



Habits, Distribution, &c. The majority of the Hirudinea 

 are inhabitants of fresh-water, and live, like the Medicinal Leech, 

 by sucking the blood of higher animals Vertebrates or Molluscs. 

 It is doubtless in correlation with this intermittent parasitism the 

 chance of finding a vertebrate host being an infrequent one that 

 the crop has attained such vast dimensions, holding, in the case of 

 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, BrancluUion occurs on the outer surface 

 of the Skate, Electric Ray, 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 Pontoldella, 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), Herpcbdella (Nephelis) 

 and the gigantic Macrcbdella. 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 



