200 STUDIES IN EVOLUTION 



region overlap each other, and are furnished with very long 

 hairs, or setae. The appendages of the head include the 

 antennae and the mouth parts, the latter consisting of the 

 mandibles and maxillae bearing palps and setae. 



The legs of the thorax have been worked out in detail, 

 and are shown on Plate VI, figures 8, 9. No essential differ- 

 ences have been observed in the series attached to the free 

 segments. Each segment bears a pair of biramous append- 

 ages, originating at the sides of the axis, as in other trilo- 

 bites (Walcott, I. c.). The anterior legs are the longest, and 

 the others gradually become shorter towards the pygidium. 

 Those which are here taken for description are the legs of 

 the second and third free thoracic segments. The entire 

 length of the legs has been exposed from the dorsal side, by 

 removing the overlying pleurae of the thorax, which con- 

 cealed nearly half their length. Each limb consists of two 

 nearly equal members, one of which was evidently used for 

 crawling and the other for swimming. These two members 

 and their joints may be correlated with certain typical forms 

 of crustacean legs among the Schizopoda, Cumacea, and 

 Decapoda, and may be described in the same terms. There- 

 fore each limb is composed of a stem, or shaft, with an outer 

 branch (exopodite) and an inner branch (endopodite). Plate 

 VI, figure 9, shows the joints of the stem (6, 7), the exopo- 

 dite (ex, 1 and 0), and the endopodite (en, 1-5). The pre- 

 cise form of the coxal joint of the stem (coxopodite) has not 

 yet been clearly made out. It is followed by a broad joint 

 about twice as long as wide, which may be referred to the 

 protopodite. 



The endopodite (figure 9, en) was the member used for 

 crawling, as in the Schizopoda. The three proximal joints 

 (5, 4-> &) are similar in form to 6, and taper gradually out- 

 ward. The distal portion is completed by two slender cylin- 

 drical joints (2, _Z), the latter bearing at its extremity short 

 setae, or bristles, of which three are commonly to be seen. 



The other member, the exopodite (ex), lies over the en- 

 dopodite. It apparently articulates with the protopodite, but 



