468 C. E. Beecher — Thoracic Legs of Triarthrus. 



these tissues in the presence of iron sulphate in solution, or 

 from the action of hydrogen sulphide as a result of decompo- 

 sition in a chalybeate water. 



It was apparent that specimens preserving organs so delicate 

 as antennae ought to show in addition other anatomical features 

 of great interest. This expectation has been fully realized in 

 the study and preparation of the collection belonging to the 

 Yale University Museum, but at present it is proposed to give 

 only a preliminary description, and to figure some of the 

 details of the thoracic legs. 



No essential differences have been observed in the series of 

 legs attached to the segments of the thorax. Each segment 

 bears a pair of biramous appendages originating at the sides of 

 the axis (Walcott). 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 concealed 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 Schizojpoda, Cumaeea, and Decapoda, and may be de- 

 scribed in the same terms. Therefore, each limb is composed 

 of a stem, or shaft, with an outer branch (exopodite), and an 

 inner branch (endopodite). Figure 3 shows the joints of the 

 stem (6, 7), the exopodite (ex, 1 and 2), and the endopodite 

 (en, 1-5). The precise details of the coxal joint of the stem 

 (coxopodite) have not yet clearly been made out. It is fol- 

 lowed by a broad joint about twice as long as wide, which may 

 be referred to fhe protopodite. 



The endopodite was the member used for crawling, as in 

 the Schizojpoda. The three proximal joints, the ischiopodite 

 (5), meropodite (4), and carpopodite (3), are similar in form to 

 the protopodite (6), and taper gradually outwards. The distal 

 portion is completed by two shorter cylindrical joints, the pro- 

 podite (2) and the dactylopodite (1), the latter bearing at its 

 extremity short setee, or bristles, of which three are commonly 

 to be seen. 



The other member, the exopodite, lies over the endopodite. 

 It apparently articulates with the protopodite, but may spring 

 from what is here referred to the coxopodite, as its basal por- 

 tion is very broad and originates close to the articulation of 

 protopodite with the coxal joint. The proximal joint of the 

 exopodite (2) is somewhat arched and tapers rapidly. It ex- 

 tends to the ends of the pleurae, and is the longest joint of 



