276 



COMPARATIVE AN ATOM V 



CHAP. 



iug no specific functions. A comparison of tlie larva with tlie adult animal sliows 

 that the body of the former corresponds Avith the anterior portion of the body of the 

 latter, i.e., in segmented animals with the head segment. The smaller portion of 

 the larva corresponds with the posterior end of the adult animal, while the trunk, 

 with the exception of the mid-gut, remains iu its embryonic condition in the larva. 

 This is again to be explained by the fact that the embryo, which is provided with 

 little or no nuti'itive yolk, must early develop organs necessary for independent 

 feeding and locomotion, and these most indispensable organs lie chiefly at the 

 anterior end of the body. This also explains Avhy the locomotory organs of the 

 larva, the ciliated rings, lie near the mouth, mostly somewhat in front of it ; and 

 why, besides this, there sometimes appears a preanal ciliated ring ; why also, in 

 certain (Polytrochan) Annelid larvfe, other segmental ciliated rings appear as the 

 segments of the trunk develop. 



Direct Embryonic Development. — This is chiefly found in fresh-water worms. 

 The embryo is provided with enough nutritive yolk, generally stored in the endoderm 

 cells, to enable it to develop direct (usually within an egg shell). Hence it follows 

 that the organs necessary for free independent locomotion and feeding as an embryo 

 are unnecessary. The comparison of the direct development of the embryo, say of 

 Lumbricus, with that of a pelagic larva, e.g. a Troeliophora, is very instructive. 



g-. 



*^ Tun- ill. 



Fig. is;.— Embryo of Lumbricus (after Wilson). Optical iiieJian longitudinal section, e, Ecto- 

 derm ; pjie, pole cells of tlie mesodenn ; m, mesodermal .streaks ; sh, rudiments of the segmental 

 body cavities in the niesodenn ; rib, neuroblast cells : hm, rudiment of the ventral chord ; vm 

 visceral layer ; jjm, parietal layer of the mesoderm somites ; iig, rudiment of the infra-cesophageal 

 ganglion ; g, rudiment of the brain ; kh, head cavity ; o, mouth ; st, stomodaeum ; md, mid-^ut • en 

 endoderm. 



The egg of Lumbricus is supplied with little yolk. The egg and the embryo 

 which develop out of it are nourished in another manner. In every capsule 

 there are several eggs in the midst of a mass of albumen which nourishes them. 

 The embryo (Fig. 187) is at a certain stage egg-shaped, and surrounded on all 

 sides by a thin unciliated ectodermal epithelium. The mouth lies anteriorly, 

 somewhat near the ventral side ; it is surrounded by an epithelial thickening, and 

 leads into a short stomodfeum. This again opens into a very spacious mid-gut, 

 whose epithelial wall lies close to the body wall all round. On the ventral side 

 only, masses of cells, the two germ streaks, to be described later, are intercalated 

 between the intestine and the body epithelium. A proctodajum is wanting, it 



