THE EVIDENCE OF THE THYROID GLAND 189 
separate are joined together in the mid-line ventrally to form a single 
terminal plate-like appendage known as the operculum. On its 
posterior surface the operculum carries the genital duct on each side. 
So also in the scorpion group, the operculum is always found 
and always carries the genital ducts. 
A survey of the nature of the opercular appendage demonstrates 
the existence of three different types— 
1, That of Limulus, in which the operculum is free, and carries 
only the terminations of the genital ducts. In this type the duct on 
each side opens to the exterior separately (Fig. 75). 
2. The type of Scorpio, Androctonus, Buthus, etc., in which the 
Gen.duct._ 2 
Gen. duct. Gen. duct. 
Fic. 75,—OpercuLtum oF LimuLvus To Fic. 76.— OpeRcULUM OF MALE 
SHOW THE TWO SEPARATE GENITAL ScoORPION. 
Docts. Ut., terminal chamber, or uterus. 
operculum is not free, but forms part of the ventral surface of the 
body-wall, but, like Limulus, carries only the terminations of 
the génital ducts. In this type the duct on each side terminates 
in a common chamber (vagina or uterus), which communicates with 
the exterior by a single external median opening. This common 
chamber, or uterus (U¢.), extends the whole breadth of the operculum 
(as seen in Fig. 76), and is limited to that segment. 
3. The type of Thelyphonus, Hypoctonus, Phrynus, and other 
members of the Pedipalpi, in which the operculum forms a part 
of the ventral surface of the body wall, but no longer covers only 
the termination of the genital apparatus. It realiy consists of two 
parts, a median anterior, which covers the terminal genital apparatus, 
