524 PROFESSOR E. RAY LANKESTER. 
in Scorpio is a pentagonal! sclerite divided into a right and 
a left half by a median groove (woodcut fig. 5, s¢ left upper 
figure). This is, in like manner, the metathoracic sternite, 
of which more will be said below. 
Fic. 5.—The seventh (op) and eighth (ga) pairs of appendages of Scorpio 
(left) and Limulus (right), together with the thoracic metasternites 
(st of the upper figures), and sternites of the eighth segment (s¢ of 
the lower figures). The anterior face of the appendages is shown 
Drawn from the object. 
Following in Limulus as in Scorpio upon the metathoracic 
sternite, is a lid-like plate, the hinge of which is transverse 
to the long axis of the body, and on the inner face of which 
are placed, both in Scorpio and in Limulus, the genital 
apertures, male or female, as the case may be (woodcut fig. 6, 
vil, right Limulus, left Scorpio). 
The history of development in Limulus shows that this 
genital operculum starts as two independent processes of 
the body, which are to be regarded as the appendages of the 
seventh segment. The operculum retains throughout life 
evidence of its double origin, and closely resembles in form 
the five succeeding pairs of appendages which carry the 
respiratory lamelle. 
In Scorpio, on the other hand, the genital operculum is 
relatively of very small size, as seen in figs. 5 and 8 go; in 
fig. 6, it and the following appendages are drawn on an 
enlarged scale for the purpose of comparison with the corre- 
sponding parts in Limulus. Very little trace of having been 
formed by the union of two lateral appendages is exhibited 
by the genital operculum of Scorpio. At the same time its 
1 Pentagonal in the subgenus Buthus, from which my drawings and 
notes are taken, but more triangular and reduced in size in the subgenus 
Androctonus. 
