42 A HISTORY OF RECENT CRUSTACEA 



• 7. The appendages of the seventh segment are dis- 

 tinguished from the two preceding pairs by the name 

 maxillipeds, meaning maxillary feet or jaw-feet, because 

 they often conspicuously combine the function of a mouth- 

 organ with the general appearance of a crustacean leg. 

 This is very much the case in the Amphipoda, where they 

 conclude the series of cephalic appendages, having here 

 something of the opercular function which they exercise 

 still more conspicuously in the Isopoda. In the Copepoda, 

 which- are content with one pair of maxillae, there are two 

 pairs of ' foot-jaws,' in regard to which the singular dis- 

 covery has been made that they belong to a single segment, 

 and yet do not violate the rule that a single segment is 

 limited to a single pair of appendages. By tracing tlie 

 process of development Dr. Claus made it clear that they 

 were in fact the exopods and endopods of a single pair of 

 limbs, which had separated so as to wear the appearance 

 of two distinct pairs, which is much as if the radius and 

 ulna in the human arm were to become independent and 

 produce a four-handed man. 



In the higher Crustacea these appendages, without 

 being divided, are only the first of a series of three pairs 

 of maxillipeds. Their forms vary exceedingly in the 

 different groups. Sometimes the endopod is seven-jointed 

 like an ordinary limb, at others the terminal part is 

 reduced to insignificance. Often the epipod and exopod 

 are important both in size and function. 



8. In regard to the eighth segment, a difficulty arises 

 as to how the appendages should best be designated. In 

 some groups, such as the Amphipoda and Isopoda, they 

 belong not only in theory but in fact to the trunk, and 

 they have in these groups been called gnathopods, a word 

 which, like maxillipeds, means jaw feet, and which was 

 chosen to indicate that they grasped the food in a jaw-like 

 manner. But in the higher Crustacea these appendages 

 practically belong to the mouth and not to the trunk, 

 their general appearance and functions allying them 

 closely to the preceding pair. Under these circumstances 

 it seems best to call them the second maxillipeds. For, 



