NILS HJ. ODHNER, PTISANULA LIMN^OIDES. 9 



the subintestinal ganglia. The visceral ganglion is soinewhat 

 remowed from those two, being situated under the intesfcine. 

 Only a trace of the prosobranchiate crossing of the connectives 

 remains in the visceral loop; the nervous system of Ptisanula 

 thus exhibits the characteristics of the opisthobranchiate type 

 and resembles that of Scaphander (cfr Pelseneer 1894). 



A comparison of the organization just described with 

 that of Diaphana (Fig. 19) gives the following results: In 

 the last-named form the nervous system is more concentrated. 

 Thus, we find that the right pleural ganglion is closely j oined 

 to the cerebral as well as to the pedal one through the fusion 

 of their respective connectives; the visceral loop further is 

 constricted to a mére ring around the pharynx, and the 

 visceral ganglion lies close to the parietal ganglia. It re- 

 presents therefore a higher stage of concentration than that 

 of Ptisanula, which on the other hand is somewhat more 

 contracted than that of Scaphander, owing to its shorter 

 cerebroparietal connectives. 



Organs of Sense. Another remarkable difference be- 

 tween Ptisanula and Diaphana exists in the reciprocal relation 

 of the optic and olfactory regions. While in Ptisanula the 

 optic nerve issues from a ganglion and a nerve-trunk, which 

 is common to it and the tentacular nerve passing to the ol- 

 factory epithelium, Diaphana exhibits another arrangement: 

 the optic nerves emerge direct from the cerebral centra and 

 are separated from the tentacular nerves and their ganglia 

 (cfr. Figs. 16, 17). In connection with the conditions named 

 there is a remarkable difference in the position and the 

 construction of the eyes. In Ptisanula they are situated im- 

 mediately beneath the ectoderm, from which they are not 

 separated by a stratum of connective tissue. The eye has 

 the shape of a ball (Fig. 14), consisting of a peripheral layer 

 of retinal cells, pigmented in their inner ends, and a central 

 vitreous corpuscle of a grainy consistency. The external side 

 of this lens, where no pigment covers it, is flattened or is 

 only slightly convex. The optic nerve enters in the hind 

 wall of the eye. The eyes are surrounded, except on their 

 front side, by connective fibres and blood lagoons. Beneath 

 them some cells are observable containing a secretion (a in 

 Fig. 14); these cells are probably chromatophores. 



