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 

 aud always carries the genital ducts. 



A survey of the nature of the opercular appendage demonstrates 

 the existence of three different types — 



1. That of Lirnulus, 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 



Fig. 75.— Operculum op Limulus to 

 show the two separate genital 

 Ducts. 



Gen. duct 



Fig. 76. — Operculum 

 Scorpion. 



Gen. duct. 



of Male 



17., 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 genital 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 ( Ut.), 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 really consists of two 

 parts, a median anterior, which covers the terminal genital apparatus, 



