5384 PROFESSOR E. RAY LANKESTER. 
escaped observation.! They are found on the posterior face 
of the median sternal elevation or lobe which unites the two 
lateral elements or appendages which go to form one of the 
double lamelligerous organs of that animal (Plate XXVIII, 
fig. 10 stg, and woodcuts, figs. 6 and 7 stg). The lips of the 
stigma are chitinised, and the opening leads into a funnel-like 
cavity with chitinised walls. The sternal integument further 
shows one or two small sclerites, the ‘‘ epistigmatic sclerites ” 
(epst), by the side of the stigma. These stigmata occur in 
the position mentioned, not only at the bases of the appen- 
dages of the four segments corresponding to those which 
carry the pulmonary stigmata in the Scorpion, namely, the 
ninth, tenth, eleventh, and twelfth, but also at the base of 
the appendages of the eighth segment, which represent the 
pectines of the Scorpion, and at the base of the genital oper- 
culum. They are connected with the attachment of a series 
of powerful muscles, the thoraco-branchials, which, taking 
their origin in the thorax, are inserted into the integument 
right and left at the base of each of the six pairs of abdo- 
minal appendages. The function of these muscles is clearly 
enough to agitate this series of plate-like organs, either for 
the purpose of respiration or fur that of locomotion, probably 
for both simultaneously. 
The fact that the insertion of a muscle into the integu- 
ment of Limulus is connected with a “‘ cupping ” of the area 
of attachment is remarkable but not without parallel. The 
series of dorsal entapophyses have a precisely similar signi- 
ficance, and in other Arachnida, e.g. Thelyphonus (fig. 12 
msg fig. 13 p, and fig. 14 m), we find an identical 
arrangement on both ventral and dorsal surface, the stig- 
mata being, however, much shallower than in Limulus. 
I am not aware of the occurrence of such “ muscular 
stigmata ”’ in any other Arthropoda than the Arachnida, at 
any rate, of stigmata comparable to those of Limulus. 
Usually the tendons of muscles are in Arthropoda formed 
by solid fibrous extensions of the subepidermic layers of 
the integument. 
The tendons or processes connected with the parabranchial 
stigmata, and with the dorsal entapophyses of Limulus, are by 
no means entirely formed by the invaginated epidermis and its 
chitinous product. The tissue below the epidermis is deve- 
loped in a very special manner, and forms part of an endo- 
skeleton which in the thoracic region gives rise to a very 
remarkable internal sternum or entosternite. The struc- 
1 Tconimunicated an account of their occurrence and probable signi- 
ficance to the Royal Society on May 26th, 1881. 
