CIECULATOEY SYSTEM OF SPIDER. 373 



orifices free from dirt, and thus prevent any obstruction to the free 

 ingress and egress of the air. 



(962.) In the Araneidae, the form and arrangement of the spiracles 

 are somewhat different. According to Treviranus, there are four pairs on 

 each side of the cephalothorax, situated immediately above the insertions 

 of the legs ; and in addition to these, there is one pair constantly found 

 on the under surface of the abdomen, and four pairs of smaller apertures 

 of less importance on its upper part. 



(963.) In order to understand the manner in which respiration takes 

 place inpulmonibranchice of the structure above described, it is necessary 

 to suppose the existence of a vascular apparatus, by means of which the 

 circulating fluid is continually spread over the laminae of the respiratory 

 sacculi, and afterwards returned to the circulation in a purified condi- 

 tion. It is true that, owing to the extreme difficulty of tracing vessels 

 of such small dimensions, the continuity of the entire system is rather 

 an inference deducible from a general review of the facts ascertained, 

 than absolutely a matter of demonstration. We will therefore briefly 

 lay before the reader the data upon which physiologists found the opi- 

 nions entertained at the present day relative to the means whereby the 

 circulation of Arachnidans is accomplished. 



(964.) According to Treviranus, spiders are provided with a long 

 contractile vessel (fig. 189, a), which runs along the mesial line of the 



Fig. 189. 



Plan illustrating the circulatory system in a Spider: a, dorsal vessel; b, suspensory muscle ; 

 c,the ocelli; d, poison-gland; e, palpus ; f, pulmonibranchial organ; g, poison-fang; A, cephalo- 

 thorax; i, caecal appendices to the stomach; I, vascular trunks derived from the dorsal heart, 

 running to the pulmonibranchise ; m, abdomen ; n, spinnerets. 



back, and resembles in form the dorsal vessel of insects, although in 

 structure it is widely different. In insects, it will be remembered, the 

 dorsal vessel communicated freely with the abdominal cavity by nume- 

 rous valvular apertures, and neither arteries nor veins were necessary for 

 diffusing the blood through the system ; but in the Pulmonibranchiate 

 Arachnidans numerous vascular trunks (II) are given off from both 

 sides of the dorsal heart and are dispersed in all directions. All the 

 branches proceeding from the sides of the dorsal vessel are presumed to 

 be of an arterial character, with the exception of a few large canals 



