102 COMPARATIVE ANATOMY 



The intestine is surrounded by a large perivisceral blood 

 sinus, circumscribed by a thin membranous wall : dorsally 

 this wall forms the floor of the pericardial sinus ; ventrally it is 

 attached to the longitudinal muscles of the body-wall ; laterally 

 it is separated from the body-wall by a considerable space, 

 which in the first eleven thoracic segments is bridged over 

 by a series of intersegmental transverse muscular partitions, 

 whose position and arrangement recall the muscular septa of 

 the worm. The cavities which they include, however, contain 

 blood, and must therefore be regarded, not as ccelom, but as 

 a number of lateral sinuses, belonging to the general system 

 of blood spaces, into which the apparent body cavity or 

 hsemocele is divided. In the abdomen there is neither 

 pericardial sinus nor lateral sinuses, and the perivisceral sinus 

 is surrounded by a so-called dermal sinus running right round 

 the body. 



In addition to the blood spaces already enumerated, we 

 may distinguish a ventral or neural sinus surrounding the 

 nerve cord, and in each limb a ventral or efferent, and a 

 dorsal or afferent sinus, having lateral connections respectively 

 with the endites and exites. 



When the muscles of the heart are relaxed in diastole, the 

 elastic tension of the alae cordis pulls its walls apart, its cavity 

 is distended, and blood is admitted by the lateral ostia. When 

 the heart contracts in systole, the valves of the ostia are closed, 

 and the blood is driven forward into the cephalic aorta and 

 the lateral vessels leading to the shell-gland. In the former 

 case the blood flows over the organs in the cephalic region, 

 thence flows backwards in the perivisceral sinus. The lateral 

 and ventral walls of this sinus are fenestrated so that blood 

 passes from it to the neural sinus and to the bases of the 

 limbs, thence flows outwards along the ventral side of each 

 limb, back along the dorsal side, enters the flabellum and 

 bract, is oxygenated in the bract, and thence is returned to 

 the body. In the thorax the blood passes in the lateral 

 sinuses to the pericardial sinus, and so back to the heart. 

 In the abdomen it enters the dermal sinus, and flows forward 

 till it reaches the last of the transverse partitions, and then 

 is directed upwards into the pericardial sinus. The blood 

 passing to the cephalic shield by the lateral vessels circulates 

 in a large sinus surrounding the shell-gland on either side; 



