26 



tlie mesoderm is derived in part from the primary ecto- 

 derm, and it is possil)le that these cells are the ones con- 

 cerned in the formation of the nervous system ; on which 

 view its origin would be but ectodermal in disguise. An 

 exception to the mesodermal origin of most of the organs 

 was noticed above. This is the case of the gonads, which 

 are stated to arise at a later stage as ectodermal ingrowths 

 ventral to the level of the lateral nerve cords. The con- 

 nection with the ectoderm is then lost, and the ducts are 

 developed later above the level of the lateral nerves. The 

 origin of the various organs has now been traced. During 

 the later part of its stay in the egg capsule the larva 

 lengthens considerably, until the little worm, now 

 about 1"5 mm. long, forsakes the protection of its 

 embryonic shelter to become an independent though 

 microscopic unit in the teeming life around its 

 birthplace. 



The development through the larva of Desor as 

 sketched above is not the only form which occurs in the 

 family of the Lineidse. In some other species of 

 Nemerteans a free swimming pelagic larva, known as the 

 Pilidium, is formed, and a slight knowledge of its develop- 

 mental history throws some light upon the peculiar forma- 

 tion of the ectoderm in Desor's larva. A typical gastrula 

 is formed, M'hich then acquires a dorsal tuft of long, fused 

 cilia and two lappets produced by ectodermal folds hang- 

 ing down laterally on either side of the mouth. From its 

 fancied resemblance to a helmet at this stage the larva 

 derives its name. 



The young worm is then developed inside the Pilidium, 

 in whose ectoderm five invaginations now make 

 their appearance round the mouth, viz., two 2)aired 

 and one anterior median unpaired. These invagina- 

 tions lose their connection with the outer ectoderm 



