x PHYLUM ANNULATA 509 



The embryo now consists of the three large megameres with a 

 number of endoderm cells, a cap of small micromeres forming an 

 ectodermal layer which is extending over the surface, with, at the 

 posterior pole, two symmetrical groups of neuronephroblast cells 

 (four in each), arid, somewhat deeper, the two mesoblast cells. 

 From each of the ten cells last mentioned new cells are given off 

 in front in such a way as to form ten rows of cells, five on each 

 side, four being derived from neuronephroblasts and one from 

 the mesoblast cell. These two sets of rows of cells constitute the so- 

 called germinal bands (g. &.). From their subsequent fate it is clear 

 that they correspond to the mesoderm bands of Nereis plus the 

 neural plate. They grow forwards, the ectoderm extending with 

 them, over the endoderm and megameres. At first they diverge 

 widely, but their anterior ends subsequently meet towards the 

 anterior end of the embryo. Later the intermediate parts of the 

 bands, originally widely separated from one another owing to their 

 divergence during growth, approach one another and meet along 

 the middle line of the ventral surface. The germinal bands give 

 rise to the nerve-cord, the mesodermal segments, and the nephridia. 

 The layer of micromeres not only gives rise to the whole ectoderm 

 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 development 

 of the pilidium larva of certain Nemerteans (p. 290). 



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 



