172 COMPARATIVE ANATOMY. 



greatly, according to the width of the fold of skin which separates 

 them. 



The olfactory organ of Polypterus is more highly developed than that of 

 any other Fish. It is not a simple sac-like involution, but consists of six 

 radially arranged compartments 1 separated from one another by complicated 

 septa, and lying round a central spindle. A transverse section has somewhat 

 the appearance of a cut orange. A short and distinct oval sac lies against 

 the olfactory organ towards the middle line, and is entirely shut off from the 

 rest of the apparatus ; it receives a special branch of the olfactory nerve. 



FIG. 142. ANTERIOR PORTION OF THE HEAD OF Polypterus. 



A, eye ; AN, AN\ anterior and posterior openings of the external nostril : +, t, t, 

 apertures of the sensory tubes. 



The mucous membrane of the nasal organ of Fishes is always 

 raised up into a more or less complicated system of folds, which 

 may have a transverse, radial, rosette-like, or longitudinal (in re- 

 spect to the cranial axis) arrangement. The branches of the olfac- 

 tory nerve are distributed on them, and they serve to increase 

 the olfactory surface. 



Dipnoi and Amphibia. The olfactory organ of Dipnoi 

 and Perennibranchiata is always enclosed within a complete 

 or perforated cartilaginous capsule lying without the cranium 

 proper (Figs. 54, NK, 143, and N), and its mucous membrane 

 is raised into folds like those of Fishes. In all the other Am- 

 phibia it becomes included within the cranial skeleton, and 

 lies directly in the longitudinal axis of the skull in front of the 

 cranial cavity. 



In Amphibia, turbinals appear for the first time (Fig. 144, 

 C, S, E] : they are processes of the cranial skeleton projecting into 

 the nasal cavity, and thus giving rise to an extension of the olfactory 

 surface. These structures, slight traces only of which are present 

 in tailed Amphibians, attain to a very considerable development in 

 Anura and Gymnophiona, especially in the latter, where the nasal 

 chamber is converted into a complicated system of spaces and 

 cavities. A main and an accessory cavity can in all cases 

 be distinguished, but more especially in the Derotremata and 

 Myctodera ; the accessory cavity, as it lies in the maxillary bone, 

 may be described as the maxillary cavity. In certain Gymno- 

 phiona this becomes entirely shut off from the main cavity, and 



1 Each compartment resembles in structure the entire olfactory sac of other Fishes. 



