12 ARKIV FÖR ZOOLOGI. BAND 8. NIO 25. 



imal ganglion but with diffuse gangliar cells beneath the 

 olfactory lamellse (Vayssiere 1870, 1906, Pelseneer 1894). 



Thus, within the Bullids, several types are represented, 

 and among them may be traced the origin of the higher 

 forms of the olfactory organs or rhinophores of the Opistho- 

 branchia. This låter development will probably have taken 

 place in the following way: 



The lamellae of the Bulla type extend to the hind lobe 

 of the cephalic disc, which becomes enrolled just as in Aplu- 

 strum, and more or less produced; the anterior lobe is subject 

 to a similar infoldation. The whole inner surface of the lobe 

 (= hind tentacle or rhinophore) is thus either strongly lamin- 

 ated (Umbrella, Tylodina, Pleurobranchcea), or has at least 

 traces of lamination. In Hermcea dendritica, Oxynoe olivacea 

 and Pleurobranchus stellatus I have seen them on sections 

 within the rhinophores; by macroscopical examination they 

 are not observable. The rhinophores of all these forms are 

 consequently homologous to the hind cephalic lobe of the 

 Bullidse. 



The lamellse on the inner surface of the rhinophores may 

 become specialized in several directions, e. g. combined to a 

 bipectinate projecting column which is enclosed within the 

 rest of the rhinophore surrounding the named lamelliferous 

 club or column like a sheath (Tethys, Tritonia, Dorididai 

 phanerobranchiatce , Dendronotus, Doto). 



From the stages of development just described processes 

 of secondary reduction or simplification may arise. Thus, 

 in a more primitive stage, the channel-shaped rhinophores 

 may grow together to conical features (Ercolania funera; in 

 E. Panceri they are still open; Vayssiere 1889) or may 

 wholly disappear (Limapontia) . 



In the higher forms of rhinophores the sheath may dis- 

 appear (as in all the Aeolididai, all the Dorididaz cryptobran- 

 chiatw, and many D. phanerobranchiate) or the lamellae of the 

 rbinophorial club may do so (as in many Aeolididoz and many 

 D. phanerobranchiate). 1 The crista interrhinophorialis in Ja- 

 nidce is perhaps to be interpreted as a remnant of the sheath. 

 The rhinophores thus represent the remaining central pillar 



1 In Corambe only two of the lamellse are present ; they are much 

 expanded and therefore cause some resemblance to an infolded rhinophore 

 (cfr Fischer 1891). 



