ON CTENODEILUS PARVULUS. 599 



and monostylos there is here only one pair of them. The 

 segmental organs lie in the head segment, one on each side of 

 the oesophagus. They are in form of two minute coiled tubes. 

 I could only see the ciliated canal and the internal opening ; 

 an external opening described by Kennel was not visible, but 

 it undoubtedly exists. 



No segmental organs are found in the other segments. We 

 do not meet with this very peculiar condition in any other 

 fully grown Annelid. The larval form of Polygordius, on the 

 other hand, exhibits in the so-called ^'Kopfnieren " a condition 

 not unlike that of Ctenodrilus. Hence we may compare the latter 

 in this respect with the Polygordius larva. Kennel^ even goes 

 further by saying that "in Ctenodrilus the excretory organ of lar- 

 val Annelids has remained as the permanent organ of excretion." 



The Nervous System. — The nervous system agrees in 

 position and structure with that of the other two species of 

 Ctenodrilus. It lies in its entire length in the epidermis, and 

 consists of a cerebral ganglion sending two commissures towards 

 the ventral surface, where they unite to form the nerve-cord. 



A similar condition of the nervous system obtains in a few 

 other Annelids mentioned by Semper,^ as well as in the 

 Gephyrea, Priapulus and Halicryptus.^ 



The parts are too small to allow of a more minute examina- 

 tion of the structure of the nervous system. As in Halicryptus 

 and Priapulus epithelial and ganglionic cells seem to merge into 

 one another, and near the cerebral ganglion, for instance, it is 

 difficult to say where the former end and the latter begin. The 

 epithelium apparently becomes several layered near the nerve- 

 cord, as well as close to the cerebral ganglion. Peripheral 

 nerves were not to be seen. 

 * Kennel, loc. cit,, p. 392. 



2 Semper, " Die Verwandtscliaftsbeziehungen d. gegliederten Thiere," ' Ar- 

 beiten aus. d. Zool. Institut, Wiirzburg,' vol, iii. According to this author 

 the following species have the nervous system lying in the ectoderm either 

 wholly or partially : — Terebella sp. of Heligoland, Terebella zostericola, 

 Hyalinaecia tubicola, Maldane sp. 



3 ScharfF, " On the Skin and Nervous System of Priapulus and Halicryp- 

 tus," ' Quart. Journ. Micr. Soc.,' vol. xxv, 1885. 



VOL. XXVII, PART 4. NEW SER. T T 



