HOMARUS AMERICANUS MILNE-EDWARDS 127 



tions as a scaphognathite or bailer in driving the water out of 

 the branchial cavity. 



The following eight, i. e., the seventh to fourteenth somites 

 form, as we have seen, the thorax. The first three pairs of 

 thoracic appendages are called maxillipeds, the last five pairs 

 pereiopods. The first and second maxillipeds are used for pass- 

 ing food, the third has mainly a masticatory function. The 

 second and third have besides a respiratory function, inasmuch 

 as they have a gill or podobranchia attached to them. The 

 epipodite is well developed in all three maxillipeds and in the 

 first it is furnished with a fold or trough for the reception of the 

 bailer. The exopodite of all three maxillipeds has a many- 

 jointed flagellum. The endopodite shows a gradual increase in 

 complication. In the first maxilliped it is two-jointed, in the 

 second four-jointed, in the third five-jointed, considerably 

 heavier than the exopodite. In this respect the third maxilliped 

 is especially interesting for it shows clearly how a monoramous 

 appendage such as a pereiopod has developed from a biramous 

 one. If the five joints of the endopodite be counted as a con- 

 tinuation of the two-jointed protopodite, then the limb becomes 

 seven-jointed and that is the typical number of joints in a 

 pereiopod. Beginning with the proximal end of the limb, the 

 joints receive the names of coxopodite, basopodite, ischiopodite, 

 meropodite, carpopodite, propodite, and dactylopodite. The is- 

 chiopodite of the third maxilliped has a comb with a row of 

 about twenty teeth along its inner edge and a brush along its 

 outer edge. Of the five pairs of thoracic legs or pereiopods the 

 anterior three are chelipeds, i. e., they have a forceps or chela 

 at their end, whereas the last two pairs end in a simple claw. 

 Each pereiopod consists of seven joints of which the first two 

 represent the protopodite and the remaining five the endopodite. 

 The exopodite has completely disappeared. An epipodite and 

 podobranchia are present in the first four and lacking in the 

 fifth pair. The joints are articulated by means of hinges. The 

 forceps of a cheliped is formed by the propodite and dactylopo- 



