Chap. XIII.] THE COCKROACH. 267 



tudinal fibres. These fibres, unlike those of the alimentary 

 canal in vertebrates, are striated. In the mesenteron the 

 chitinous layer is absent, and the epithelial layer specially 

 modified. 



The Heart and Circulation. The heart is a long delicate 

 tube lying beneath the terga of the thorax and abdomen. It is 

 divided into thirteen segments three thoracic, and ten abdom- 

 inal ; but some of these are inconspicuous, and can hardly be 

 made out in the common cockroach, At its posterior end each 

 of the more conspicuous segments dilates into a median dorsal 

 and two lateral lobes. The lateral lobes conceal paired lateral 

 inlets. A pear-shaped mass of cells, hanging just in front of 

 the constricted opening between any two segments, serves as a 

 valve to close at once this opening and those of the lateral 

 inlets. The heart lies in a pericardial chamber, the floor of 

 which is constituted by a membrane in which are small oval 

 openings. To this membrane fan-shaped muscular bands (alary 

 muscles) arising from the anterior margin of each tergum, are 

 attached. 



The heart contracts in rhythmical waves from behind for- 

 wards ; in a young cockroach, shortly after the ecdysis they 

 have been reckoned at eighty per minute. The blood thus 

 driven forwards passes by a slender aorta along the dorsal sur- 

 face of the gullet, and terminates with a trumpet-shaped orifice 

 just in front of the nerve-ring. The blood then passes through- 

 out the body by a system of lacunar spaces. It therefore bathes 

 the various organs, and is not injected into them ; hence their 

 diffuse form, seen in the salivary glands, caecal tubes, and 

 Malpighian tubules, in the reproductive organs, and in the fat- 

 body which occupies the peri-visceral space or ccelom. The 

 blood (which is colourless, with large nucleated amoeboid cor- 

 puscles), after bathing these organs, reaches the pericardial 

 cavity through the fenestrated membrane. It is doubtful 

 whether there is any distinction into venous and arterial blood 

 a distinction perhaps rendered unnecessary by the tracheal 

 system, 



