574 SPECIAL PHYSIOLOGY. 



second are the maxillce, which support the palpi. By these parts, the food is 

 seized, examined, or even divided. In many Insects and Crustacea, and in 

 Spiders, one or more pairs of the limbs are also employed in conveying food to 

 the mouth ; sometimes, as in the crabs and lobsters, such limbs are enor- 

 mously developed, the pincers on one side of the body being smooth, and on 

 the other, knobbed. In certain perfect insects, the food being viscid or fluid, 

 the mandibular appendages are specially modified, as e..g., in the butterflies 

 and moths, in which they form a long tube or canal named the proboscis, which 

 can be unfolded from its spiral coils and protruded into flowers ; a sucking 

 proboscis also exists in certain flies and gnats. In the fleas and bugs, the 

 mandibles are penetrating and suctorial. Amongst the Annelides, the sand- 

 worms have soft feeble tentacles ; but the earth-worms and leeches have the 

 mouth either simply suctorial, or cutting and suctorial. In some worms, a 

 retractile proboscis exists, developed from the lining membrane of the pharynx, 

 and not from the cephalic segments of the exo-skeleton, like the jaws of the 

 higher Annulosa. The Annuloid Entozoa either have a special suctorial ap- 

 paratus, or live by general imbibition. The marine worm-like forms are sucto- 

 rial, whilst the Rotifera have a ciliated disc, which creates a vortex in the 

 water. In the starfishes and echini, there are no prehensile tentacula ; in the 

 Crinoida, the arms may act prehensively ; in the holothurida or sea-cucum- 

 bers, large labial appendages or tentacula exist. 



Coeknterata. These exclusively aquatic animals have contractile non-ciliated 

 tentacles, sometimes few and simple, or divided, as in the hydra, sometimes 

 very numerous, as in the sea-anemones, and often of great length and of ir- 

 regular form, as in the medusa? and others. These are always prehensile ; but 

 food may also be drawn into the body, by the alternate expansion and contrac- 

 tion of its muscular walls. In the Physograda, the mouth is developed into 

 depending tubular suctorial processes or cirrhi. 



Protozoa. In the Infusoria, the cilia draw the water and food into the buc- 

 cal orifice, and there is no other prehensile apparatus. In the lowly-organized 

 Rhizopods and Amoebae, the soft body is merely applied to the substance serv- 

 ing as food. In the Spongida, currents of water are drawn through numerous 

 incurrent or inhalant orifices, into the interior of the porous mass, whilst ex- 

 current or exhalant orifices, fewer in number, serve for their expulsion. This 

 process not only assists in respiration, but also in entangling food against their 

 sarcodous substance. Finally, the parasitic Gregarinida live by direct imbi- 

 bition. 



The Teeth and Mastication in Animals. 



True teeth, or calcified organs, belonging to the exo-skeleton, and composed 

 of dentine, or of this with enamel and cement, are peculiar to the Vertebrata ; 

 for the so-called teeth or denticles of certain Mollusca, Annulosa, and Annu- 

 loida, have no such structure. 



Teeth are entirely absent in Birds ; but they are generally, though not uni- 

 versally, present, in Fishes, Amphibia, Reptiles, and Mammalia. In the last- 

 named Class alone, are the characteristic milk teeth met with, that first tem- 

 porary and deciduous set, which falls out and is succeeded by the permanent 

 teeth. With the exception of a few fishes, and the vegetable-feeding iguanas 

 amongst reptiles, which have grinding teeth, these organs in Fishes, Amphibia, 

 and Reptiles, are essentially prehensile, or incisive, being used for seizing, and 

 holding the prey, or for dividing it into portions small enough to be swallowed ; 

 but it is in the Mammalia, that mastication proper, performed by teeth set in 

 movable jaws, is most perfect, the food being, in many of them, not only seized, 

 but afterwards gnawed or chewed. 



In the different classes of the Vertebrata, the teeth differ remarkably in 

 number, shape, position, and mode of insertion. 



Mammalia. Amongst these, the Monotremata are almost edentulous, or 

 destitute of teeth, for the echidna has no such organs, but merely horny pro- 

 cesses on the tongue and palate, whilst the ornithorhynchus has horny teeth. 

 In the Cetacea, two genera have calcified teeth before birth only, the upper jaw 

 afterwards supporting the whalebone plates. In the nianis, or pangolin, and 



