NILS HJ. ODHNER, PTISANULA LIMN^OIDES. 11 



root. The optic nerve is long and ascends to the superficial 

 eye, the tentacular nerve descends and becomes almost im- 

 mediately bifurcated into one före and one hind branch, which 

 innervate the cephalic lobe and the sides of the head. No 

 diffuse ganglious layer and no expansion of the nerval root 

 to a tentacular ganglion are noticeable (Fig. 15). 



In Doridium, Philine and Scaphander the optic and ten- 

 tacular nerves start separate from one another, though they 

 are together. Doridium alone has long eye nerves, the others 

 have very short ones. The tentacular nerve has no proximal 

 ganglion and its distal ends lead to an olfactory organ, that 

 is distinct only in Doridium. In Scaphander it is very rudi- 

 mentary (Vayssiére 1879). 



In Gastropteron the optic nerve emerges from a ganglion 

 common to it and the tentacular nerve, as in Ptisanula; it 

 is of considerable length; the tentacular nerve ramifies in the 

 cephalic disc; olfactory organs are absent (Vayssiére 1879). 

 Thus, among the Tectibranchia the variation is consi- 

 derable as regards the nerves and the exteriör equipment of 

 the olfactory organs. Where these begin to appear, as in 

 Diaphana and Ptisanula, the sensory epithelium is uniformly 

 and simultaneously distributed to the under-side of the ce- 

 phalic disc and to the sides of the head. In this arrangement 

 we may find the origin of the various kinds of olfactory 

 lamelhe in the Bullidse, they being in some forms placed 

 laterally on the head, in others on the under-side of the 

 cephalic lobe. In the first case they are usually bipectinate 

 and resemble the rhinophores of the Nudibranchia. They 

 have been described in Haminea (Vayssiére 1879, Pelseneer 

 1894) and Atys (Vayssiére 1906). In the second case they 

 are present, either as transversal simple lamellae in Bulla 

 (Vayssiére 1885, 1906), or as a pectinate projection like a 

 paucifoliate gill in Aplustrum (Vayssiére 1906 and my own 

 observations). Both in Bulla and in Aplustrum the folds are 

 placed between the före and the hind lobes of the cephalic 

 disc. In Aplustrum these lobes are produced in tentacles and 

 infolded like channels. 



The nervous system in all Bullidse shows a long optic 

 nerve leading to a superficial eye and a tentacular nerve 

 separated from it, but placed quite close, without any prox- 



