CHAPTER VI. 



NEMERTINI. 



The Nemerteans laj their eggs enclosed in gelatinous enve- 

 lopes, either singly or balled into large masses of spawn. It 

 appears that fertilization may take place either outside or 

 inside the body of the female. In the latter case the sperma- 

 tozoa penetrate into the female genital organs (egg sacs) 

 through their efferent ducts. In many forms (Monopora 

 vivipara) the eggs are there developed up to the maturity of 

 the embryo. The development takes place either directly or 

 by means of a metamorphosis. The latter is of various kinds, 

 according as a free-swimming larva differing very much from 

 the ultimate shape of the animal is produced, or merely a 

 larval form which does not differ essentially from the young 

 animal, but which nevertheless produces the latter within 

 it. In the first case, in view of the shape of the larva, one 

 speaks of a Pilidium larva, in the latter case, of development 

 after the type of Desor, thus named from its discoverer. 



I.— Development through the Pilidium Larva. 



As the result of the equal cleavage a regular blastula arises 

 from the egg of Lineus lacteus. It loses its regular shape, 

 owing to a considerable increase in the size of the cells of 

 the lower half and to the occurrence at the same time of a 

 flattening on the under-side of the blastula (Fig. 102 A). 

 The outer and inner germ-layers can be distinguished on the 

 blastula as early as this, for the cells of the ectoderm are 

 smaller than those of the entoderm. The first trace of the 

 middle germ-layer is likewise already present. In the cleav- 

 age cavity and in contact with the entoderm are found a 

 number of cells (Fig. 102 A) which to all appearances take 

 their origin from the entoderm (Metschnikoff, No. 20), and 



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