APLYSIA. 225 



RESPIRATORY SYSTEM. 



Aeration of the blood is brought about through the 

 medium of the single asymmetrically placed ctenidium, whose 

 position and external structure have already been described 

 under External Characters. For its blood supply and innerva- 

 tion see the sections on Blood Vascular System and Nervous 

 System respectively. 



In microscopic preparations the ctenidium shows a regular 

 epithelium, in some parts ciliated, in others, especially near 

 the insertion of the ctenidium, strongly glandular. Beneath 

 the epithelium lies a loose tissue consisting of connective tissue 

 and muscle fibres. The centre of each pinnule is a blood 

 space whose walls are bounded by the tissue just mentioned. 

 There is no capillary system, but the afferent branchial vessel 

 opens by " gaping orifices " into this cavity, and the efferent 

 vessel collects in a similar way. 



EXCRETORY SYSTEM. 

 (Figs. 14, 15, 21.) 



The excretory system of Aplysia consists of a single 

 asymmetrically placed kidney situated on the left side of 

 the body, overlying the heart and visceral hump, and covered 

 by the shell and mantle. In shape it forms a triangle whose 

 base is directed forwards over the heart, the apex of the 

 triangle curving towards the right posteriorly. It will be 

 noted that the kidney has separated itself from the visceral 

 mass, and lies close under the mantle. In all Euthyneura 

 the single kidney represents the morphologically right (topo- 

 graphically left) member of an original pair of which the 

 fellow has entirely disappeared in Aplysia. 



The kidney is a coelomic sac lined by a glandular epithe- 

 lium which is produced inwards in the form of parallel lamellae. 

 The renal sac communicates with the pallial cavity and with 

 the pericardium. The former opening, the renal pore, is the 



