480 ZOOLOGY SECT. 



and at [the same time the germinal bands grow forwards (D), 

 diverging as they go, and take up a position beneath the 

 margin of the ectoderm-cap : as the latter extends its area 

 they converge anteriorly, and thus furnish it with a thickened 

 margin, the bands themselves being in contact in front and 

 behind, and divergent in the intervening region, so as to 

 enclose a nearly circular space. The ectoderm-cap, accompanied 

 by the germinal bands, now grows over the megameres, finally 

 enclosing them completely by uniting on the ventral surface. 

 This process is obviously one of epibolic gastrulation. 



The ectoderm of the embryo gives rise, as usual, to the epidermis 

 of the adult as well as to the stomodaeum (E, F,mtk.) and procto- 

 da?um. From the germinal bands are formed the ventral nerve-cord 

 and the nephridia. The endoderm arises from small cells budded off 

 from the megameres, which gradually grow round what is left of 

 the latter. The remains of the megameres thus become enclosed 

 in the enteron of the embryo and undergo gradual absorption, 

 serving simply as food, and not giving rise to any part of the 

 tissues. The cocoon contains no albumen, and the yolk of the 

 megameres supplies the whole of the nutriment required by the 

 embryo up to the time it leaves the cocoon. The young is 

 hatched at a comparatively advanced state of development, and, 

 after escaping from the cocoon, adheres by its suckers to the body 

 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, 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 development of the 

 Pilidium larva of certain Nemerteans (p. 273). 



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 ffirudo 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, Hcvmopsis vorax, taken in, when young, by 



