129 317 



possible that I have made the same mistake, with regard to the male sex as earlier 

 authors with regard to the female sex, viz. the mistake of describing some [)ai-ls of 

 the retrocerebral organ as parts of the brain; the extreme smallness of the objects 

 must here be my apology. In the Pohjarthra male I have not been able to see any 

 trace of the brain. Neither a peripheric nervesystem nor an oesophageal nor a pedal 

 ganglion shown in the female sex in several species (e. g. in Callidina and Discopns 

 by Zelinka; in Hydatina by de Beauchamp and Martini (1912, p. 612) and in 

 Conochiloides by Hlava (1905, p. 209) have been found in the males; bul the neces- 

 sary methods of investigation have not been used. 



Sensitive organs. Female. As mentioned above the most typical sensitive 

 organs of the Rotifers are two pairs of tactile organs, Ihe anterior and posterior 

 lateral organs, which are connected by long nervethreads either directly witli Ihe 

 brain or with peripheric ganglia-cells, lying laterally to the brain. 



Only in a few cases both the two anterior and two posterior lateral organs are 

 far removed from each other. This is the case with Asplancimadœ and ApsUiis, and 

 they are then connected with each other by means of a transversal commissure; in 

 the Synchu'tadce the two anterior ones meet each other, but in most of the Rotifera 

 they coalesce to a single so-called dorsal organ, or dorsal antenna. Commonly two 

 nerves are present, more rarely only one. It lies in the middle line dorsally, and is 

 only rarelj' asymmetric {Rattiilidæ, Gastropus). The cuticula is often elevated to a little 

 hill, on the top of which a bunch of sensitive hairs, rarely only one single hair, is 

 visible. In some cases, especially in loricate Rotifers, the hill is converted into a real 

 antenna often situated immediately behind the wheel-organ, is drawn in together 

 with the latter, and is the first which makes its appearance when the wheel-organ 

 is again unfolded (^Brachionidœ, Anurœadœ, Salpinadœ). In the Bdelloidu it is segmen- 

 ted and provided with muscle threads, and may here as well as in some of tlie 

 Meliceriidœ reach a length of about one third of the body {Cephcdoziphon); in Eos- 

 phora Hirschfelder (1910, p. 69) has interpreted the organ as a combination of 

 a tactile and a static organ. The dorsal organ is very rarely absent {Conochihis). The 

 posterior or ventral lateral organs hardly ever coalesce (perhaps in Copeiis caiidatus) ; 

 they are almost always widely separated lying on the borders between the dorsal 

 and ventral side and are provided either with a bunch of sensitive hairs or one 

 single hair; they have not been ascertained in the Bdelloidu. In the tubedwelling Rotifers 

 they are often placed ventrally, situated upon long antenna-like organs, which are 

 placed anteriorly so that their tips are the first part of the body which appears 

 when the animals make their appearance outside the edge of the tube. 



In the males these sensitive organs are constructed in accordance with those 

 of the females; here however they are very difficult to detect; especially in many of 

 the strongly reduced plancton Rotifers they are almost always unknown, and it is a 

 question if they really exist here. In the Asplanchna the}' are identic in botli sexes, 

 and in many males, e. g. those of the genus Syiichu'ta, Hiidalimi, Gastropiis hiiptopiis 

 and others two nerves to the common dorsal organ are present. It is especially the 



D. K. D. Vidensk. Selsk. Skr., naturv. og mathein. Afd,. 8. Række, IV. 3. 40 



