LIMULUS AN ARACHNID 547 
interlacing of their respective tendons. In the figure, 
nothing but the skeletal structure, cleaned of its muscles, is 
represented. 
In Scorpio, a similarly shaped loose skeletal piece is 
ound, which gives attachment to muscles in the same way 
and has a similar relation to the. ventral nerve-mass and 
artery, by which in Scorpio it is perforated (Pl. XXVIII, 
figs. 6 and 6 a). The corresponding body in Mygale is 
(Pl. XXVIII, figs. 5,5 a) more closely similar in form to 
that of Limulus than is that of Scorpio. 
In order to make a close comparison of these Entoster- 
nites, it will be necessary to determine exactly the insertions 
of the muscles to which they give origin; and further, to 
ascertain how far the histological structure of those of Scor- 
pio and Mygale agrees with that of Limulus. The results 
of this investigation I hope to make the subject of a future 
publication. In the meantime the close correspondence in 
general character of the three Entosternites figured on 
Pl. XXVIII cannot escape notice, and fully justifies the 
importance which Straus Durkheim attached to them. The 
two pairs of tendinous outgrowths right and left of the 
central plate in Limulus correspond with the three pairs 
seen in Mygale, whilst the deep anterior notch in the latter 
corresponds with the shallower excavation in Limulus, in 
which the number 7 is placed in the drawing, and in which 
in the animal itself the bend of the alimentary canal is 
placed, the mouth being actually below the central region 
of the plate, so that the alimentary canal passes first for- 
wards beneath the plate and is then reflected so as to pass 
backwards whilst resting on the upper surface of the plate. 
‘Whilst of this as of so many other structures of the 
Arachnida (such as the lung-books, &c.) which have been 
compared in the present memoir with structures in Limulus, 
a renewed and critical examination is absolutely needed, 
yet we have sufficient ground, even in our present 
incomplete knowledge, for concluding that the agreement 
as to them presented by the two animals is a very close 
one. 
In no Crustacean is a free entosternite at all similar to 
the organ under discussion known. The apodemes of the 
sternal surface of Decapodous Crustacea do not resemble it 
in form though of a similar function. The nearest approach 
to it is seen in the rod-like skeletal organ found in the 
abdomen of Lepidoptera, and described by Leydig (‘ Bau 
des Thierischen KGrpers,’ Atlas, pl. vi, fig. 1). Its 
shape and position are very different, however, from the 
