456 



LECTURE XIX. 



attenuating to the base of the abdomen.* The pulmonary sac {figs. 

 168, 170, h), which receives the air by the anterior respiratory ori- 

 fice, is of an elliptical form ; the vascular surface is augmented by a 

 number of bi'oad and close-set lamellge which project into its interior. 



In the scorpion {fig. 164), the stigmata or pulmonary orifices are 

 eight in number, four on each side of the under surface of the an- 

 terior broad segments of the abdomen (1, 2, 3, 4). They have 

 the form of oblique fissures, surrounded by a thickened margin, to 

 which the name of" peritrema" has been given. The vascularlining 

 membrane of the cavity adheres to this margin, and is at first 

 simple, but afterwards gives attachment to a series of twenty 

 broad and close-set lamellae, arranged, as in the spiders, like the 

 leaves of a book. The genus Phrynus has only two pairs of pulmo- 

 nary sacs ; but each sac has eighty lamellae. 



The peculiar organs of secretion in the class Arachnida are those 

 which 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. Tlie 

 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 

 envenomed sting. The glands which supply this weapon with its 

 poisonous 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.f A minute slit 

 may be observed near the point, which is the 

 common outlet of two slender ducts, that gra- 

 dually dilate into two secreting sacs, lodged in 

 the cavity of the expanded part of the joint, 

 and separated from each other by a double ver- 

 tical partition. Their chief tunic is formed by 

 a layer of smooth muscular fibres, external to 

 which is a stratum of cylindrical cells. 



The poison-apparatus of spiders is placed at 

 the opposite extremity of the body. The perfo- 

 rated sting or fang forms the second joint of the 

 mandible or modified antenna, upon which it 

 has a gynglymoid movement, and lies concealed 

 and protected, when not in use, in a furrow with 

 dentated margins upon the basal joint {fig. 

 165, m'). The poison gland {fig. 169, a) is an elongated ovoid vesicle, 

 the exterior of which is characterised by spiral folds produced by the 



169 



Mygiile. 



* XXIV. p. 535. 



t Prep. No. 2161. 



