356 THE ANATOMY OF INVERTEBRATED ANIMALS. 



the abdominal wall which surrounds it on all sides, leaving 

 only a small pericardial space. 1 Beyond the slender aortic 

 canal in which the heart terminates anteriorly, and which 

 passes into the thorax and the head, no vessels appear to be 

 given off from the heart. 



Delicate triangular sheets of muscular fibre, the alary 

 muscles, are attached in pairs by their bases to the wall of 

 the pericardial chamber, while their apices are inserted into 

 the hypodermis. They occupy the interspaces left by the 

 principal dorsal branches of the tracheae, which form arches on 

 each side of the heart. 



From the inner face of the abdominal wall, processes are 

 given off, some of which appear to hang freely into the ab- 

 dominal cavity, while others accompany the numerous tracheae 

 which pass to the alimentary canal. When the abdominal 

 cavity is laid open, its inner lining has a villous appearance, 

 and often seems to be full of free granular matter, as the pro- 

 cesses very readily break up into fragments. The substance 

 which thus fills up the interspace between the parietes of the 

 abdomen and the contained organs is the corpus adiposum. 

 It is made up of cells often so arranged as to form a network, 

 and it usually has a milk-white color, which arises partly from 

 the air contained in the trachea?, and partly from innumerable, 

 strongly refracting granules contained in its component cells. 



There are ten stigmata on each side of the body of Blatta^ 

 eight in the abdomen, and two in the thorax. The latter are 

 situated between the prothorax and mesothorax, the meso- 

 thorax and the metathorax, respectively ; above the attach- 

 ment of the coxae and beneath the terga. The abdominal 

 stigmata lie in the soft integument which connects the sterna 

 and terga of the somites. All the stigmata are situated in 

 conical thickened elevations of the integument. The thoracic 

 stigmata are the largest, and the anterior pair have a distinctly 

 two-lipped aperture, the anterior lip being notched in the cen- 

 tre. The openings of the abdominal stigmata are more oval, 

 and are inclined backward. Immediately within each stigma 

 the tracheal trunk into which it opens is provided with a val- 

 vular arrangement, by which the passage can be closed or 

 opened. 



1 Cornelius (" Beitrage zur naheren Kenntniss von Periplaneta (Blatta) ori- 

 entals," 1853) found that the pulsations of the heart could readily be observed 

 in Blott-ae which had recently undergone ecdysis. They were as frequent as 

 eighty in the minute ; but allowance must be'made for the disturbed condition 

 or the insects under observation. 



