XL] THE ALIMENTARY SYSTEM. 435 



while the aperture of the windpipe is situated below behind 

 the tongue (Fig. 370, g). 



The teeth have been already noticed in the Seventh Lesson 

 of this course. 



On each side, the mouth is bounded by a fleshy cheek, 

 only slightly distensible. 



3. In reviewing the mouth as it exists in Vertebrates gene- 

 rally, we find that it may be entirely destitute of lips, as in 

 Birds and Reptiles. These structures are not, however, con- 

 fined to man's class, as they exist in some Fishes, e.g. the 

 Carp and Dory. Nevertheless, lips are mainly mammalian 

 structures, and may attain a much greater development than 

 in man. Thus, in the Right-Whales the lower lip is an enor- 

 mous structure, rising up on each side and overlapping the 

 slit-like fissures which separate the numerous baleen plates 

 before described. 



The lips may be much more extensible than in man, as in 

 even so nearly an allied form as the Orang. The upper lip 

 may unite with the nose to form an elongated proboscis, as 

 in the Elephant. It may be medianly divided by a vertical 

 separation, as in the Rabbit, the Cat, or the Camel ; and 

 each lip on each side may send a process inwards, behind 

 the incisors, as in the Rat. Lips, by rare exceptions, may 

 be absent even in man's own class, as e.g. in the Ornitho- 

 rhynchus, and they are scarcely developed in the Dolphins. 

 An exceptional condition is seen in the Sturgeon, where an 

 exceedingly small mouth opens on the under surface of the 

 head, its lips being protrusible less by their own structure 

 than by the singularly elongated and relatively large sus- 

 pensorial apparatus before noticed. 



In the Marsipobranchs we find a great circular lip, destined 

 for suction and supported by complex special cartilages ; and 

 finally, in the Lancelet we meet with an altogether exceptional 

 structure, namely, a mouth in the form of a vertical fissure, 

 and provided on each side with a series of long, slender, 

 jointed and ciliated 1 tentacles. 



Sensitive tentacles, numerous and of small size, may 

 border the lip, as in the Lamprey ; or the lip may send out 

 six or eight long tentacles, as in the Myxinoids. Higher 

 Fishes may as in the Mullets and Siluroids have fleshy 

 and sensitive labial barbs, or cirri. 



A soft palate is a structure peculiar to man's own class, with 



i Ciliated structures are such a* bear cilia, or minute filaments which keep up 

 an incessant waving motion. (See " Elementary Physiology," Lesson VII- 3.) 

 F F 2 



