THE ARTHROPODA 101 



through the body to open at the anus. The posterior end 

 of the intestine is lined by a chitinous reflection of the external 

 cuticle, and is therefore known as a proctodseum. 



The body of Apus is not divided up internally into a series 

 of compartments corresponding with the external annulation 

 as is that of the earthworm, and the space surrounding the 

 gut, though capacious, is not a ccelom, but an extensive system 

 of blood spaces or sinuses. There are, however, many indica- 

 tions of an internal segmentation. The heart, the musculature 

 of the trunk and the gonads, are segmented in correspondence 

 with the external annulation, and the nervous system is on 

 the segmental plan, but corresponds with the series of limbs, 

 and not with the segmentation of the body. 



The heart lies in the mid-dorsal line in the thoracic region. 

 It is a straight, muscular, thin-walled tube, divided by as 

 many constrictions into eleven segments, but its cavity is 

 continuous i.e. is not divided into chambers by internal 

 dissepiments. Its most anterior division lies in the first 

 thoracic segment, and its ten succeeding divisions lie in the 

 corresponding segments of the thorax behind the first. 

 Posteriorly the heart tapers off and ends blindly in the 

 eleventh thoracic segment, but anteriorly it is continued 

 forwards as the so-called cephalic aorta, which widens out in 

 the head region to form a large cephalic blood sinus sur- 

 rounding the digestive gland. At the junction of the heart 

 and the cephalic aorta, two wide vessels are given off right 

 and left in the segment corresponding to the second maxilla. 

 These vessels pass to the cephalic shield, and supply the 

 shell gland with blood. There are no other definite blood 

 vessels. 



The heart lies in a considerable pericardial space, and 

 is attached to the walls bounding this space by a number of 

 segmentally arranged convergent bundles of elastic connective 

 tissue threads, the alae cordis. The pericardial space of the 

 crustacean must not be confused with that of a mollusc. It is 

 not a part of the ccelom, but contains blood, and the heart 

 communicates with it by eleven pairs of valvular openings or 

 ostia corresponding to the first eleven pairs of thoracic 

 segments. The valves of the ostia are so arranged that they 

 admit blood from the pericardial space into the heart, but 

 prevent any flow in the reverse direction. 



