340 MISS E. J. BECK ON THE MUSCULAR 



The second appendage, chela (n), is large and composed of six sclerites. As in 

 the first appendage the penultimate sclerite forms with the last a powerful chela ; 

 between the coxal sclerites of this pair is the remarkable upper lip, the camero- 

 stome (cam), on the dorsal surface of which is a small chitinous sclerite. 



The third and following appendages of the prosoma each consist of seven sclerites 

 and two small terminal claws ; these appendages are the walking-legs. 



On the coxal joints of the second, third, and fourth appendages there are small 

 processes which meet in the median line and assist the mouth as mandibular organs ; 

 these are the " sterno-coxal processes." The coxal joint of the fifth appendage is 

 fused to that of the sixth, and is immovable without it. 



To the sterno-coxal process of the third appendage is attached a movable "epi- 

 coxite " similar to the pieces so named on limbs 2, 3, 4, and 5 of Limulus 1 . 



The coxal sclerites of the second, third, and fourth appendages of one side meeting 

 those of the other side in the median line have almost obliterated the sternite of the 

 prosoma ; all that remains is a small pentagonal sclerite situated in the median line 

 between the coxal joints of the fifth and sixth appendages of either side. This is 

 called the " thoracic metasternite " (met), and is the exact equivalent of the chilaria of 

 Limulus. (See Lankester, loc. cit.) 



The appendages of the mesosoma are much modified. In an early state of develop- 

 ment a rudimentary pair of appendages appears on each of the six segments. These 

 afterwards disappear from view, with the exception of the first and second pairs. The 

 first pair are simply small plates which together form the genital operculum (go) as in 

 Limulus. The second pair, the pectines (pec), are carried on a small sternal sclerite, 

 and are comb-like organs with a number of lamellae set on their inferior margin. 



On the sternal surface of the four last segments there are wide sclerites which 

 apparently carry no appendages ; on closer examination two stigmata will be seen on 

 each sclerite ; these lead into small sacs, which are, according to Professor Lankester, 

 nothing more or less than the appendages themselves invaginated, that is, completely 

 pushed outside in. The appendages are composed of an axis, on which are set a number 

 of lamellae, like the pages of a book ; they perform the function of respiration, and 

 exactly represent the four posterior branchial appendages of Limulus in an introverted 

 condition. 



The six sternal sclerites of the metasoma have been mentioned above as fused to the 

 tergites, and as being, in the last five segments, of a cylindrical form. These segments 

 carry no appendages. 



For a more detailed description of the skeleton of the Scorpion the reader is referred 

 to a paper by Prof. E. Pay Lankester, " Limulus an Arachnid " (Quart. Journ. of Micr. 

 Science, 1881). 



[' The epicoxites of Limulus and Scorpio appear to be similar in character to the rudimentary second ramus 

 of the limbs of Scolopendrella and other forms noted by Wood-Mason.— E. E. L.] 



