296 ANIMAL BIOLOGY. [Part II. 



the intestine, and almost at once splits into two trunks a pos- 

 terior aorta, which follows the course of the intestine on its 

 convex side, and an anterior aorta which, after giving off branches 

 to the genital organs, passes into the body-cavity, where it may 

 readily be traced, giving off branches to the salivary glands and 

 mid-region of the foot, and proceeding to the head, piercing the 

 nerve-collar (Fig. 86, A., ao.), and being distributed to the buccal 

 mass and neighbouring parts. The ultimate branches of the 

 arteries would seem to end in funnel-shaped dilated openings into 

 the lacunae which lie within and around the tissues, there being 

 no true capillaries. The blood finds its way into large blood- 

 sinuses, there being a large lateral sinus on each side of the foot. 

 There is also a large visceral sinus running along the concave 

 side of the body whorl. This latter is in direct communication 

 with the circular pulmonary sinus which runs round the floor of 

 the pulmonary chamber; but the communication between the 

 lateral sinus and this pulmonary circle must be indirect. From 

 the pulmonary circle afferent pulmonary vessels lead the blood to 

 the roof of the lung-chamber, whence efferent pulmonary vessels 

 form factors of the large pulmonary vein (p. v.), which conveys 

 the blood to the auricle. It will be seen that the renal organ 

 (ren.) is in the course of the blood which is finding its way to the 

 pulmonary vein, on the right side of the roof of the lung. 

 The blood is colourless, and contains nucleated corpuscles. 



Respiration. The afferent and efferent pulmonary vessels in 

 the roof of the pulmonary chamber form a network, in which 

 the blood is separated from the air by the thin partition of their 

 walls. Here, therefore, carbonic acid gas is given off by the 

 blood and oxygen absorbed. Air enters and leaves the lung 

 by the pulmonary aperture, being drawn in by the contraction 

 of the muscular fibres of the septum which forms the floor of 

 the chamber, and being expelled by the relaxation of these 

 fibres and consequent rise of the septum by the pressure of the 

 viscera which lie beneath' it. 



The Renal Organ. The renal organ consists of two parts, the 



