APLYSTA. 223 



into the large body cavity in which lie the gut and its glands, 

 the nervous system and the reproductive organs. This cavity, 

 the apparent body cavity, is therefore not a true coelom, for it 

 contains blood, but a haemocoele. Only the cavities of the 

 pericardium, of the renal sac, and of the hermaphrodite gland 

 are coelomic. 



The portion of the haemocoele into which the blood from 

 all parts of the body, with the exception of the kidney, and the 

 ctenidium, is discharged is known as the ventral abdominal 

 sinus. Like the lacunae in the tissues this sinus has no 

 bounding epithelium. From this sinus the blood is sent either 

 to the kidney or to the ctenidium on its way back to the 

 heart. To facilitate the propulsion of the blood, venous 

 sinuses with muscular walls collect it from the common sinus 

 and empty themselves into the kidney or the ctenidium. These 

 muscular sinuses are not extensive, but occur only in close 

 connection with the kidney and ctenidium. Their walls are 

 fenestrated. Five of these sinuses constitute the main afferent 

 branchial sinus, the remainder form the renal sinus. 



The chief factors of the afferent branchial sinus are : — 



1. A small sinus (Fig. 14, sin. 1) receiving blood from 

 the left anterior body wall. It runs backwards along the edge 

 of the sub-pallium on the left side of the body, ventral to the 

 kidney and pericardium, and posterior to the ctenidium bends 

 to the right to enter the main sinus. 



2. A large sinus (sin. 2) opening from the ventral 

 abdominal sinus (anterior part) into the left of the main 

 afferent branchial sinus. 



3. A large sinus (sin. 3) bringing blood from the posterior 

 part of the ventral abdominal sinus, viz., from the visceral 

 hump. It enters the main sinus on its posterior wall. 



4. A small sinus (sin. 4) from the right body wall. 

 Through its fenestrated walls it receives also from the ventral 

 abdominal sinus. 



