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 cox 
of the feet, is a lamina, which may be considered as sternum, formed 
of the union of four pieces. ‘The abdomen of the Scorpions, of 
Phrynus and Telephonus and (among st the Arachnoidea trachear ta) 
of Obistum is divided into rings; in others, as the spiders, it is 
without rings, and has a much softer iintpountent 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, 
Phrynus, in some Epeire (Epetra cancriformis), &e. 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 lamelle 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. 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 Patni the 
intestinal canal forms a wide sac, which has in front, on each side, 
five blind appendages that become wider towards the extremity, of 
which 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 Jaxodes, Acarus amertcanus (or crenatus KoLuar) in Trevrra- 
nus, Zettschr. fiir Physiol. tv. 2. 1832. 
