126 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



primitive crustaceans, and apparently the appendage of the trilobite satisfies the expectation. 

 It is well, perhaps, as a test, to consider whether by modification this limb could produce 

 the various types of limbs seen in other members of the class. In the first place, it is 

 necessary to have clearly in mind the peculiarities of the appendage to be discussed. 



It should first of all be remembered that the limb is articulated with the dorsal skeleton 

 in a manner which is very peculiar for a crustacean. The coxopodite swings on a sort of 

 ball-and-socket joint, and at the outer end both the exopodite and the basipodite articulate 

 with it. Since the exopodite articulates with the basipodite as well as with the coxopo- 

 dite, the two branches are closely connected with one another and there is little individual 

 freedom of movement. This is, of course, a necessary consequence of their articulation 

 with a segment which is itself too freely movable to provide a solid base for attachment 

 of muscles. The relation of the appendifer, coxopodite, and two rami is here shown dia- 

 grammatically (fig. 33), the exopodite branching off from the proximal end of the basipo- 

 dite at the junction with the coxopodite. 



In all trilobites the endopodite consists of six segments, and the coxopodite of a single 

 segment the inner end of which is prolonged as an endobase. There does not seem to be 

 any variation from this plan in the subclass, although individual segments are variously 



Fig- 33- Diagrammatic representation of an 

 appendage of the anterior end of the thorax of 

 Triarthrus bccki Green, to show relation of exo- 

 podite and endopodite to each other and to the 

 coxopodite. Much enlarged. 



modified. The exopodites are more variable, but all consist of a flattened shaft with setae on 

 one margin. No other organs such as accessory gills, swimming plates, or brood pouches 

 have yet been found attached to the appendages, the evidence for the existence of the vari- 

 ous epipodites and exites described by Walcott being unsatisfactory (see p. 23). 



In the Ostracoda the appendages are highly variable, but it is easily seen that they 

 are modifications of a limb which is fundamentally biramous. In most species, both exop- 

 odite and endopodite suffer reduction. The exopodite springs from the basipodite and that 

 segment is closely joined to the coxopodite, producing a protopodite. In some cases the 

 original segments of the endopodites fuse to form a stiff rod. While highly diversified, 

 these appendages are very trilobite-like, and some Ostracoda even have biramous antenn;c. 



The non-parasitic Copepoda have limbs exceedingly like those of trilobites. Many of 

 them are biramous, the endopodites sometimes retaining the primitive six segments. Coxop- 

 odite and basipodite are generally united, and endopodite and exopodite variously modified. 

 Like some of the Ostracoda, the more primitive Copepoda have biramous antennae. 



As would be expected, the appendages of the Cirripedia are much modified, although 

 those of the nauplius are typical. The thoracic appendages of many are biramous, but both 

 branches are multisegmented. 



In the modern Malacostraca the ground plan of the appendages is biramous, but in most 

 orders they are much modified. In many, however, the appendages of some part of the 

 body are biramous, and in many the endopodites show the typical six segments. From the 

 coxopodites arise epipodites, some of which assist in swimming, and some in respiration. 



