740 



From this we may see, that the spiraciilar sense organ shows 

 itself to belong to tiie lateral line system of epidermal sense organs, 

 which is generallv also understood l).y the term mucns-organs. The 

 direct [»roof has not yet been given, but may perhaps be found in 

 stages earlier than those which I have ^-ludied. 



The ram. olicus, in all the studied embryos, arises with a ganglion 

 like thickening from the buccal ganglion of the facial nerve. 



In the 39^. m.m. long embryo, it runs along the cartilage of the 

 ear capsule — but not yet surrounded by the cartilage — dorsally 

 and caudalwards. It sends off a few thin branches to the organs in 

 the lateral hue canal of the regio otica, and a thick branch, which 

 goes to the spiracular sense organ aci'oss the jugular vein. 



In the 98 m.m. long embryo, a part of the ram. olicns is over- 

 grown by the cartilage of the ear capsule. This is also the case with 

 the Ganoids. Contrary to the Selachians the sense organ itself is 

 -surrounded by cartilage in both Ganoids and Dipnoi. 



We shall now |)ass on to the 63 m.m. long embryo of Heptanchus. 

 The small external opening of the spiracle is here siliiated far back- 

 waids. The lissure like opening in the gut reaches still further 

 rostralwards. If we accept that the beginning — the base — of 

 the veslibulum falls on the section which passes through the anterior 

 margin of this fissure, tiien the lop of the vestibuliim lies slill 28 

 sections further forwai'ds. In this top the sense organ bladder 

 opens without an excretory duel. It can be traced in 12 sections 

 rostralwards, along the auditory organ, from which it is separ- 



Fig. 3(/. Cross section through the otic 

 region of the skull of a 63 m.m. long 

 embryo of Heptanchus cinereus. 



Fig 'Sb shows the spiracular organ 

 under tiigher magnification. 



