122 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



ferments, part of the fibrin being converted into 

 peptones ; and on starch that of salivary fluids. 

 As in many other Invertebrata, the term hepatic has 

 been applied to regions of the digestive tract, rather 

 on account of the brown coloration of these regions, 

 than from the definite experimental knowledge that 

 their secretions have in any way the functions of 

 a human liver. Some starfishes are capable of pro- 

 truding the oasophageal portion of their intestine, and 

 of engulfing prey, which they then draw into their 

 bodies. 



It cannot be too much insisted on that one of the 

 most prominent characteristics of the Artliropoda 

 is the conversion of one or more pairs of its appendages 

 to the service of the mouth ; they become, in fact, 

 mouth-organs (gnathites), and are, from a physio- 

 logical point of view, to be regarded as part of the 

 digestive apparatus. 



There is, perhaps, no investigation which can be 

 more interesting than the study of the modifications 

 undergone by these parts, whether we examine a 

 single individual, such as the lobster, with its six 

 pairs of mouth organs, or extend our survey over 

 the whole series of arthropod ous forms ; in the one 

 case we observe the modifications undergone by 

 similarly constituted parts as they take on different 

 parts in the duty of performing a common function, 

 and, in the other, we see a multitude of changes, con- 

 ditioned by differences in affinity and in habit. The 

 remarkable phenomena associated with the parasitic 

 mode of life of some members of this phylum will be 

 considered later on. (See page 179.) 



When we examine a lobster or a crayfish, we find 

 that six pairs of appendages enter into the service of 

 the mouth, and that in most of them we can make 

 out the leading points in organisation, which are cha- 

 racteristic of a " typical " appendage. (See page 301.) 



