AND BNDOSKELETAL SYSTEMS OF LIMULUS. 319 



Below the outermost layer of chitin are the flattened epidermic cells which produced 

 the cuticle ; these are surrounded by pigment, or contain pigment. 



In the case of the network on the inner surface of the carapace, the layers of chitin, 

 except the outermost, are continued, surrounding spaces filled with connective tissues. 

 The tubes piercing the layers are more or less filled with connective tissue-cells. 



III. Appendages. 



1. Prosomatic Appendages — Of the six pairs of prosomatic appendages, the five hinder 

 pairs are more or less alike (the walking-legs), the last being used for digging as well 

 as walking. The first pair is much smaller and has fewer joints. 



The proximal joint (coxa) of a walking-leg is a short piece, widening out from its 

 distal end dorso-ventrally till it becomes very wide at its attachment to the body. 



Attached to the coxa of the third, fourth, and fifth pairs is a small movable piece, 

 described by Lankester as the epicoxite, and directed towards the middle line. 



The coxa itself, where it projects below the floor of the thorax, is strongly toothed ; 

 this portion is the sterno-coxal process, and is used for manducatory purposes. 



The sterno-coxal process of the sixth proximal appendage is not toothed but is 

 slightly roughened. 



When the base of a walking-leg is looked at from within the body (see PI. LXXII. 

 and PI. LXXVI. fig. 7) there are seen, rising almost vertically from the sterno-coxal 

 process, two narrow chitinous bars, at first diverging from one another, so as to form 

 an anterior and posterior border to this portion ; to these borders various muscles are 

 attached. After running nearly parallel for a short distance, and inclined outwards, 

 they converge and meet in a slightly thicker piece ; from the posterior end of this a 

 short thick bar rises upwards and backwards, whilst from its anterior end another piece 

 goes upwards and forwards to a hioh, which articulates with the " coxal pivot " on the 

 " sternal convexity." From this a rod goes backwards to meet the anterior short bar ; 

 the part where they meet is a rounded knob, into which the principal coxo-tergal muscle 

 is inserted. Other smaller bars go from the anterior border to this articular " knob." 



Each of these sets of chitinous bars may be termed an '■'■ entocoxite." 



The first thoracic apjendage differs from this in that there is but a single chitinous 

 rod passing upwards, forwai-ds, and outwards from the coxa along the membranous 

 sternal region in front of the camerostome, at the side of which the coxa is situated. 



This single rod probably represents the posterior border of the other entocoxites, 

 judging from the insertion of its muscles. The entocoxite is not fixed to any hard 

 structure at its upper and anterior end, and in a line with it are two small sclerites. 



2. Mesosomatic A2)pendages. a. Gill-plates. — Of these there are five pairs, each pair 

 being united across the median plane. 



The appendage consists of a bag, flattened antero-posteriorly, open to the mesosomatic 

 VOL. XI. — PART X. No. 2. — May, 1885. 3 c 



