260 LECTURE XIX. 



out the abdomen ; another enters the cephalo-thorax, and resolves 

 itself into groups corresponding in number with the limbs to the 

 extremity of which the fine silvery tracheae can be traced. The pul- 

 monary sac (^figs. 112, 113.6), which receives the air by the an- 

 terior respiratory orifice, is of an elliptical form ; the vascular surface 

 is augmented by a number of broad and close-set lamellae, which pro- 

 ject into its interior. 



In the scorpion, the stigmata or pulmonary orifices are eight in 

 number, four on each side of the under surface of the anterior broad 

 segments of the abdomen. They have the form of oblique fissures, 

 surrounded by a thickened margin, to which the name of " peritrema " 

 has been given. The vascular lining membrane of the cavity adheres 

 to this margin, and is at first simple, but afterwards gives attachment 

 to a series of broad and close-set lamellae, arranged, as in the spiders, 

 like the leaves of a book. 



The peculiar organs of secretion in the class Arachnida are those 

 Avhich prepare the material of the web, which is analogous to silk, and 

 those which secrete the venomous liquid. The former are proper 

 to spiders ; the latter common to both spiders and scorpions. The 

 modification of the abdominal segment of scorpions, by which its 

 hinder half is converted into a slender, jointed, flexible, tail-like ap- 

 pendage, seems to have special reference to the wielding of the en- 

 venomed sting. The glands which supply this weapon with its poi- 

 sonous fluid are lodged in that well-known pyriform dilatation formed 

 by the last joint of the tail, and which is terminated by the slender 

 sharp, recurved sting.* A minute slit may be observed near the point, 

 which is the common outlet of two slender ducts, that gradually dilate 

 into two secreting sacs, lodged in the cavity of the expanded part of 

 the joint, and separated from each other by a double vertical partition. 



The poison apparatus of spiders is placed at the opposite extremity 

 of the body. The perforated sting or fang forms the second joint of 

 the mandible or modified antenna, upon which it has a gynglimoid 

 movement, and lies concealed and protected when not in use in a 

 furrow with dentated margins upon the basal joint {Jig. 109. m). 

 The poison gland is an elongated ovoid vesicle, the exterior of which 

 is characterised by spiral folds produced by the arrangement of the 

 fibres of the contractile tunic. The duct traverses the basal joint of 

 the mandible and the cavity of the fang, and terminates in a fissure on 

 its convex surface near the point. In the true AranecB, the Clubiones, 

 and the Lycozce, the poison glands extend into the cephalo-thorax ; 

 but in the bird-spiders {Mygale) they are limited to the mandibles. 

 It is probable, therefore, that the effects of a wound occasioned by 



* See Prep. No. 2161. 



