XV NEMATODA 455 



tendency to produce cuticle, both have a simple, straight alimentary 

 canal devoid of appendages, and both possess a single layer of 

 longitudinal muscles and no circular muscles. The excretory organ 

 of Chaetognatha must be regarded as the coelomic cavity. This is 

 suppressed in Nematoda, but a similar phenomenon is seen in 

 Arthropoda, which are undoubtedly derived from the coelomate 

 Annelida, and the suppression of the coelomic cavity may be con- 

 nected with the excessive production of cuticle. The excretory cells 

 of Nematoda have no known homologues in Chaetognatha and the 

 early development is very dissimilar. That of Chaetognatha is of a 

 very primitive type, whilst that of Nematoda is excessively specialized. 

 In one feature alone do they resemble one another, i.e. in the early 

 differentiation of the genital cells. 



Perhaps a more thorough knowledge of the normal development 

 of IsTematoda would enable us to regard it as a modification of that 

 of Chaetognatha, but for the present the question must be left in 

 abeyance. 



LITERATURE REFERRED TO 



Boveri, Th. Die Entwicklung von Ascaris megalocephala mit besonderer Riicksioht 

 auf die KernVerhaltnisse. Festschrift zuni 70ten Geburtstag von Garl. von Kupfi'er. 

 Jena, 1899. 



Boveri, Th. Die Potenzen der ^scarfs-Blastomeren bei abgeanderter Forderung. 

 Festschrift zum 60ten Geburtstag Richard Hertwigs. Jena, vol. 3, 1910. 



Hertwig, O. Vergleioh der Ei- und Samenbildung bei Nematoden. Arohiv fiir 

 mikroscopische Anatomic, vol. 36, 1890. 



Miiller. Beitrag zur Embryonalentwickhmg der Ascaris megalocephala. Zoologica, 

 Heft 17. Stuttgart, 1903. 



Zur Strassen. Embryonalentwicklung der Ascaris megalocephala. Arohiv fiir 

 Entwicklungsmechanik, vol. 3, 1896. 



Zur Strassen. Die Geschichte der T-Riesen von Ascaris megalocephala. Zoologica, 

 Heft 60. Stuttgart, 1906. 



