TRACHEATA 313 
les just behind the cephalic shield, gives off a median and two 
lateral vessels ; the former supplies the head and mouth parts, 
the latter form a ring round the oesophagus and unite to form 
a ventral vessel which lies above the nerve cord. These vessels 
open into lacunae, which ultimately communicate with the 
body-cavity, and from this the blood returns to the heart 
through paired valvular openings in each chamber. The heart 
is supported by a pericardium, to which are attached certain 
muscles, the alae cordis; these arise from the body-wall. The 
blood is colourless, and contains numerous white corpuscles. 
A pair of stigmata are found on the third, fifth, eighth, 
tenth, twelfth, and fourteenth leg-bearing segments, situated on 
the soft tissue between tergum and sternum, immediately 
behind the base of each leg. Each stigma opens into a swollen 
sac, which gives off two main tracheae and a number of smaller 
ones, which supply the neighbouring parts. The arrangement 
of the larger branches varies in the anterior and posterior ends 
of the animal, but two main branches are usually present, one 
running dorsally, the other ventrally ; they divide into smaller 
and smaller branches, and ultimately form a network which 
ramifies through every organ of the body. ‘The tracheae are 
kept open by a spiral thickening of their chitinous lining. 
The ventral nerve cord bears sixteen ganglia, each of which 
gives off three nerves on each side to the legs and adjacent 
muscles. The first of these supplies the poison claws; the 
cord is connected by circum-oesophageal commissures ending in 
the bilobed cerebral ganglion which gives nerves to the eyes, 
antennae, and mouth organs. When a basilar segment is pre- 
sent, the corresponding ganglia are fused together. In some 
Chilopoda the first few ganglia of the ventral nerve cord are 
more or less completely fused; some writers have held that 
this indicates a division of the segments of the body into 
thorax and abdomen. Besides the eyes, sensory hairs are found 
on the antennae, and in Scwtigera there is a peculiar sense organ 
on the ventral part of the head consisting of a pouch lined with 
sensory hairs, each of which is connected with a nerve fibril. 
Lithobius is dioecious, and the generative organs usually 
attain their full size during the spring. The testis consists of 
a blind tube, the lining cells of which give rise to the sperma- 
