540 PHILOSOPHY OF ZOOLOGY. 



divided into a number of lobes, which may be regarded as 

 so many separate organs. The spermatic ducts are two in 

 number, varying greatly in length and the number of tubes, 

 which, like roots, combine in their formation. They unite, 

 in general, into a common duct near the penis ; but, pre- 

 vious to doing this, they are joined by the ducts of the ve- 

 siculae seminales, and other accessory tubes, which, in dif- 

 ferent genera, exhibit a great diversity of character. In 

 this case, the penis is usually a simple tube, protected 

 in some insects at the base by two scales, which sepa- 

 rate upon entering the vagina of the female, and thus pre- 

 pare a passage for the penis, and serve likewise for reten- 

 tion. In many of the crabs and spiders, the seminal ducts 

 continue single, each opening into a separate penis. The 

 external opening is usually situate at the extremity of the 

 abdomen, beside the anus. In the libellulae, or dragon- 

 flies, it is seated at the base of the abdomen, and in the 

 spiders, at the extremity of the palpi. 



The males are seldom of so large a size as the females > 

 and frequently exhibit peculiar characters in their abdo- 

 men, eyes, or antennae, by which they may be distin- 

 guished. 



The female organs exhibit fewer varieties of structure in 

 the different genera than those of the other sex. The 

 ovaria consist of numerous tubes, in which the eggs are 

 prepared. These open, in insects, into a common oviduct, 

 terminating in the vulva. Previous to the termination of 

 the oviduct, it receives the ducts of one or more vesicles. 

 The crabs have two oviducts and two vulva? *. 



The female may, in general, be distinguished from the; 

 male, by the superior size of the abdomen. In some cases, 

 both males and females survive the process of generation, 



* See Vol. I. p. 418. 



