i6o COMPARATIVE ANATOMY 



Malpighian tubules arise as diverticula from the proctodaeum, 

 which itself is an invagination of the outer integument. They 

 must therefore be regarded as invaginations of the epiblast, 

 and in spite of their different situation and non-segmental 

 arrangement they can hardly be denied the character of 

 nephridia which have lost their internal openings on the 

 disappearance of the coelom. 



The liver casca of the cockroach may fairly be homologised 

 with the digestive glands of the crayfish, both being out- 

 growths of the mesenteron, but the salivary glands of the 

 cockroach have no homologue in Astacus. These glands 

 are developed as diverticula of the stomodsum, and consist 

 of two lobulated glands and a thin-walled sac, the salivary 

 receptacle, lying on either side of the oesophagus. The 

 ducts of the two glands of each side unite and run forwards 

 as a single duct which Joins the similar duct of the other 

 side below the nerve cord in the neck. The ducts of the 

 two salivary receptacles similarly unite to form a median duct 

 which opens into the common sahvary duct in the head, and 

 the conjoined duct opens into the buccal cavity behind the 

 lingua. 



The circulatory apparatus consists of a median contractile 

 dorsal vessel, the heart, and a system of blood sinuses ; there 

 are no tubular vessels or very definite blood channels as in the 

 crayfish, unless the blood channels running in the nervures 

 of the wings may be accounted as such. The heart itself 

 extends through the whole length of the thorax and abdomen, 

 and is divided into thirteen chambers, three thoracic and ten 

 abdominal, the two posterior chambers being very small. The 

 chambers are separated from one another by deep constrictions, 

 and the narrow passage between adjacent chambers is guarded 

 by a valve which only permits a forward flow of the blood. 

 Each chamber communicates with the pericardial space by a 

 pair of lateral ostia opening into its hinder end. The anterior 

 chamber is continued into the head as a slender tube, the 

 so-called aorta, which runs forward on the dorsal surface of 

 the oesophagus, and ends in front of the perioesophageal nerve 

 ring in a funnel-shaped ^orifice through which the blood is 

 discharged into a blood sinus. The chambers of the heart 

 contract successively from behind forwards, the waves of con- 

 traction succeeding one another so rapidly that the hinder 



Digitized by Microsoft® 



