ARACHNIDS. 559 



the thorax consists of a lamina, in which sometimes, more or less 

 obviously, four sutures or grooves are seen, which proceed obliquely 

 from the feet to the center, and indicate the original composition of 

 the thorax of four pieces. On the under surface, between the coxse 

 of the feet, is a lamina, which may be considered as sternum, formed 

 I of the union of four pieces. The abdomen of the Scorpions, of 

 Phrynus and Telephonus and (amongst the Arachnoidea trachearia) 

 of OUsium is divided into rings; in others, as the spiders, it is 

 without rings, and has a much softer integument than the cephalo- 

 thorax. On the whole, the external covering of the Arachnids is 

 soft and very extensible ; the skin is horny and hard in Scorpio, 

 Fhrynus, in some JEpeirce (Epeira cancriformis) , &c. Two layers 

 may usually be distinguished in the skin ; the external is firmer, 

 sometimes cellular, often provided with sinuous folds ; the internal 

 consists of a finely granular substance or of very delicate fibres, be- 

 neath which is a layer of pigment. In Mygale, these pigment cells 

 are very apparent. The external membrane exhibits many concen- 

 tric rings with spots between them, which, under the microscope, 

 present a deceptive resemblance to the corpuscles and lamellae of 

 bone. 



The intestinal canal of the arachnids proceeds without tortu- 

 osities to the posterior extremity of the body. Not in all of them, 

 however, is the anus situated at the posterior extremity, but in the 

 Acari more forward on the ventral surface 1 . In Phrynus and 

 Scorpio, the intestine is a narrow canal, nearly of the same width 

 throughout. In all the other Arachnids, the intestinal canal has 

 many protrusions or blind appendages of very different form ; and 

 in many Acarina these appendages are ramified, reminding us of 

 the form of the intestine in Planarice and Distomata. In the Pyc- 

 nogonida blind appendages proceed from the stomach, even into the 

 shear-shaped feelers and the feet. In the genus Phalangium the 

 intestinal canal forms a wide sac, which has in front, on each side, 

 five blind appendages that become wider towards the extremity, of 

 jwhich. the last pair is longer than the rest, and descends to the 

 ^termination of the intestinal canal. Between these appendages are 

 still twelve others smaller and vesicular, and at the hindmost part 



1 See the figure of Ixodes, Acarus americanus (or crenattis KOLLAR) in TREVIRA- 

 KDS, Zeitschr. fur Physiol. iv. i. 1832. 



