RESPIRATION AND CIRCULATION 0? ARACHNIDA. 213 



the neighbourhood of the anal opening, that we find the secret- 

 ing glands of the silky matter ; and also the spinnerets, by 

 whose aid several of the Arachnida weave for themselves webs, 

 which are often very extensive and of extreme delicacy. 



747. The respiration of the Arachnida is aerial, like that of In- 

 sects, and is sometimes effected by means of tracheae ; but among 

 the greater part of these animals, and especially amongst the 

 Spiders and Scorpions, it is concentrated in certain vesicles lodged 

 in the abdomen, and called pulmonary sacs. These last organs 

 have in their interior a multitude of membranous plates, arranged 

 like the leaves of a book : and they thus bear a stronger resem- 

 blance to internal gills, than to true lungs. Each sac receives the 

 air by an opening situated on the lower side of the abdomen ; and 

 there are sometimes two, sometimes four, or even eight of these 

 stigmata. Certain Arachnida possess at the same time both pul- 

 monary sacs and tracheae; the Segestria are thus formed. 

 Others, such as the Pseudo- Scorpions and Mites, are provided 

 with tracheae only. These tubes have the 



same structure as in Insects ; and the air ai 

 penetrates to them by two very small 

 stigmata, situated on the lower side of the 

 abdomen. 



748. The blood is white among all the 

 animals of this class. The pulmonary 

 Arachnida are furnished with a complete 

 circulating apparatus. The heart, situated 

 on the back, has the form of a long vessel, 

 and gives origin to different arteries ; the 

 blood after having traversed the body is 



returned to the pulmonary sacs, and arrives FI G--MO.-HEARTOF A SPIDER : 



. a, border of the abdomen ; 



at the heart after following a course Similar c, heart ; ar, large artery, 



to that which it traverses in the Crustacea rio^extrfmit 

 (ANIM. PHYSIOL. 292). Amongst the nar y vessels. 

 Arachnida whose respiration is effected only by the aid of tracheaa, 

 the apparatus for circulation is but little developed ; there appears 

 to be only a simple dorsal vessel, without arteries or veins. 



749. The Arachnida lay eggs like Insects ; and the male 

 differs in general from the female in the form of its maxillary 



