Introduction to Animal Morphology. 367 



uric acid concretions. In Scorpions the long, eight- 

 chambered, tubular heart lies in a pericardiac sinus, 

 suspended by alar ligaments or muscles, which help it 

 to dilate ; into each chamber venous blood enters 

 through a pair of ostia protected by valves ; out of the 

 heart, in front and behind, pass arterial continuations 

 of the heart, the anterior aorta, to the cephalothorax 

 and limbs, the posterior to the abdomen ; lateral ar- 

 teries also arise from the heart. A supraspinal branch 

 of the anterior artery runs along and supplies the 

 nerve cord. Some organs seem to have capillaries, 

 but the colourless blood also enters the insterstitial 

 lacunae, from whence it is collected in a sinus, and 

 distributed to the air vessels by lacunae, with no 

 walls, and from thence it is poured into the pericar- 

 diac sinus. 



Spiders have an abdominal heart, with three 

 pairs of ostia, but no pericardial sinus nor capillaries ; 

 the anterior aorta bifurcates into lateral branches ; the 

 posterior ends abruptly by a free mouth. Pycnogo- 

 num has a two- (or three-) chambered heart, with two 

 pairs of ostia. Mites and Tardigrades are acardiac. 



Respiration takes place by the dermis in many 

 Mites, but usually by tracheae, which are tubes placed 

 symmetrically, opening on the surface by lateral, 

 transversely oval holes [stigmata]^ and carrying air 

 into the interior of the body. These tubes consist of 

 a connective basis, continuous with the dermis, lined 

 by a chitinous cuticle, w^hich, by its elasticity, keeps 

 the tube open. When well developed, this lining 

 thickens into a spirally-coiled thread, here and there 

 interrupted by the sacculation of the tube. The stig- 

 mata are surrounded by chitinous rings, and usually 



