IV 



VERMES— ONTOGENY OF THE TVORMH 



275 



and forms the perniaueut outer integument of the young N'emeHian, The outer, 

 thinner, wall then forms inside the PUidiauL integument a sac -like covering for 

 the young Xcmci-tiun, the amnion. This integument with ciliated ridge and neural 

 plate, i.e. the primary ectoderm, falls away together with the amnion Avlien the young 

 Xoncrtian issues from the PiUdium. At an early stage we find on the inner side 

 of the ectoderm sac a layer of cells, which in the PiUdium arise out of the ecto- 

 dermal wall of the sac itself, but in the related Desoi-'s larva are said to be derived 

 from the mesoderm cells which lie in the primary body cavity. The 4 layers of cells 

 thus arising represent the rudiment of the definitive mesoderm. The central nervous 

 system arises out of 2 ectodermal outgrowths which unite to form the brain in the 

 young Xcmciiian. They grow out posteriorly into two strands, the lateral nerves. 

 The proboscidal apparatus rises firstly out of an invagination of the ectoderm above 

 the brain, and secondly out of parts of the mesoderm which surround this in- 

 vagination. 



The PiUdium larva takes up a position intermediate between the young Tur- 

 hcUarian larva of the Pohjclada {cf. p. 167, Mailer's larva) and the typical Trochophoran 

 larva. It agrees with the former in the absence of a proctodteum. The i 



Fig. isij.— .4, B, C, D, Four diagrammatic transverse sections ijeMnd the mouth through a 

 Pilidium larva during metamorphosis, to illustrate the method of formation of the mesoderm 

 and secondary (definitive) ectoderm, e, Larval ectoderm ; ei, definitive ectoderm ; a, amnion 

 ectoderm ; es, ectoderm sac ; m, mesoderm ; Z, lateral lobes of the larva ; ind, mid-gut ; ml, larval 

 mesoderm. 



rudiments of the mesoderm probably answer to the 4 mesoderm masses of the 

 yoimg Polyclad larvffi or embryos. 



A Nemertiicn larva related to the Pilidiam, Desor's larva, shows the larval 

 characteristics less developed. The ciliated ring and the ectodermal thickening with 

 the ciliated tuft which corresponds with the neural plate are wanting. But the 

 definitive ectoderm is formed, as in Pilidium, of discs which detach themselves from the 

 primary ectoderm. Many XcinerUna develop without metamorphosis. 



If we glance over the larval forms of worms we see that all are distinguished 

 by peculiarities of structure which are explained by the circumstance that the 

 animals feed independently and swim about freely at a very early stage of develop- 

 ment. The simplest organs of locomotion which can be developed early are the 

 cilia. They appear in all larva;. Ciliated rings are universally present ; the most 

 constant is the preoral, which is provided with a special nervous system (nerve 

 ring). Almost everywhere we find a spacious larval body cavity filled with fluid. 

 The larvas are hydropic, their specific gravity being nearly that of water. They 

 are provided with a functional hollow enteron and other functional parts — nervous 

 system, sensory organs, muscles, excretory organs. It is pretty generally found 

 that those parts of the larval body which function at an early stage are thrown off 

 or reabsorbed at the end of larval life, and that the organs of the adult animal consist 

 of cell material which is present in the larva as undifferentiated germ material perform- 



