8 ARKIV FÖR ZOOLOGI. BAND 8. N:0 25. 



aperture; it is furnished with glandular walls in its distal 

 part and with thin walls in its proximal end where it issues 

 from the hermaphrodite gland. In the centre of this the 

 spermatozoids are produced and so are the ova in its peri- 

 pheral layer; both of the genital products are comparatively 

 large, the spermatozoids measuring about 210 [A in length and 

 the ova 110 \i in diameter. The sperma was mature and la y 

 stored up in the upper part of the spermoviduct. 



The Nervous System. By means of reconstruction of 

 sections the following organization has been found for Ptisa- 

 nula (Fig. 18). The nervous ring is situated in front of the 

 pharyngeal bulbus. The cerebral ganglia are placed above 

 and at the sides of the pharyngeal cavity and eonnected 

 together by a commissure on the upper side of it. They 

 send nerves forwards to the mouth, the sides of the head and 

 the cephalic disc. Alongside them there is a special ganglion 

 from which the optic nerve emerges. From the same gang- 

 lion is also innervated the epithelium situated on the under- 

 side of the cephalic lobe. Among the connective tissue below 

 this epithelium there is a layer of diffuse gangliar cells; these 

 indicate, together with the fact that the epithelium itself is 

 somewhat folded, the presence of an olfactory sense-organ. 



On the under-side of the cerebral ganglia connectives 

 emerge to the pedal ganglia; these are joined together by a 

 double commissure below the pharynx. At their outer and 

 posterior sides the statocysts are situated. Details of their 

 constructions were not to be observed in Ptisanula, but in 

 Diaphana they contain a great number of ovate statoliths. 



From the hind side of the cerebral ganglia there issue 

 short connectives running back to the pleural ganglia, which 

 are also joined on to the pedal ones. A pair of long com- 

 missures also start from the hind side of the cerebral centra 

 and run to the buccal ganglia, which are situated behind the 

 pharyngeal bulbus and under the oesophagus. 



From the pleural ganglia the pleurovisceral connectives 

 continue in the backward direction. In them the parietal 

 ganglia are differentiated. The right one is the largest and 

 is found at the side of the pharynx somewhat higher than 

 the left one, that being smaller and lying beneath the oeso- 

 phageal coil. Both ganglia are situated at the sides of the 

 buccal centra. We recognize in them the supraintestinal and 



