CHAPTER XII. 



Arachnida. 



THE Arachnida include the scorpions and spiders, and as the 

 former are tolerably uniform in colour, our remarks will be 

 confined to the latter. 



The thorax is covered with a horny plate, while the abdomen 

 only possesses a soft skin, and neither show any traces of segmenta- 

 tion. From the thorax spring four pairs of logs, and a pair of 

 palpi, or feelers. Immediately beneath the skin of the abdomen 

 lies the great dorsal vessel, which serves as a heart. This vessel 

 is divided into three chambers, the general aspect of which is 

 shown in Fig. 9, Plate VIII., taken from Gegenbaur's Comparative 

 Anatomy.* 



From this heart the blood passes by vessels to each of the limbs, 

 the palpi, etc., as offsets from the double-branched aorta. The shape 

 of this dorsal vessel is peculiar, and its importance in respect to 

 colouration will be immediately apparent. 



The primary scheme of colouration in the Arachnida seems to be 

 the distinguishing of the cephalothorax from the abdomen by a 

 different colour. Thus, of the 272 species of British spiders repre- 

 sented in Blackwell's work,f no less than 203 have these parts 

 differently coloured, and only 69 are of the same hue, and even in 

 these there is often a difference of tint. So marked is this in certain 

 cases that the two parts form vivid contrasts. Of this cases are 

 given in the following list. 



Cephalothorax. Abdomen. 



Eresus cinnabarinus, Black, Bright Red. 



Thomisus floricolens, Green, Brown. 



cinereus, Brown, Blue. 



trux, Red, Brown. 



Sparassus smaragduhis, Green, Red and yellow. 



* Elements of Comparative Anatomy, by C. Gegenbaur. Translated by Jeffrey 

 Bell and Ray Lankester, 1878, p. 285. 



t Spiders of Great Britain and Ireland, J. Blackwell. Bay. Soc, 1861. 



