156 ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



auriculo-ventricular aperture are prolonged on the ventricular side 

 into two valvular folds. Below, the ventricle terminates in a wide 

 aorta, \\'hich immediately gives off a 'large branch backwards to the 

 hepatic and generative organs ; then becoming much convoluted, it 

 runs forwards along the intestine and stomach, passing between the 

 latter and the pedal ganglia (fig. 6), and finally terminating, without 

 much alteration in its diameter, in the buccal mass. 



As it passes over the pedal ganglia it gives off a considerable branch, 

 " the pedal artery," downwards to the foot ; and this pedal artery, just 

 before it enters the foot, gives off a long and delicate " metapodal " 

 branch (w'), which passes backwards, parallel to the aorta, and finally 

 terminates in the metapodium, figs. 2 and 3'. 



The mode of ending of the pedal artery is very remarkable, and 

 physiologically speaking, almost unique (fig. 6). Having entered the 

 foot, it ends suddenly, without narrowing, in a truncated open ex- 

 tremity (j/). In the living animal this open end possesses the power 

 of contracting to a very great extent, so as almost to become closed ; 

 and its condition must necessarily exercise a very considerable influ- 

 ence upon the direction and rapidity of the animal's circulation. 



Firolo'ides^ then affords the most complete ocular demonstration of 

 the truth of M. Milne-Edwards's views with regard to the nature of 

 the circulation in the Mollusca, that can possibly be desired. The 

 perfect transparency of the creature allows the corpuscles of its blood 

 to be seen floating in the visceral cavity between the intestine and the 

 parietes, and drifting more or less rapidly backwards to the heart. 

 Having reached the wall of the auricle, they make their way through 

 its meshes as they best may, sometimes getting entangled therein, if 

 the force of the heart has become feeble. From the auricle they may 

 be followed to the ventricle, and from the ventricle into the aorta, 

 whence they pass, some forwards, to the buccal mass, in which the 

 aorta ends, and through whose tissues it pours them ; some down- 

 wards, to pour out of the widely open end of the pedal artery, flooding 

 the tissues of the propodium ; and a small proportion passes directly 

 backwards to the visceral mass and to the metapodium. 



Respiratory System. — So delicate a creature would hardly seem to 

 need any special system of this kind, and I found no trace of such 

 organs in any, even the freshest and most uninjured specimens. 



In the nearly allied species Firola Keraudrenii, however, the gills 

 appear as a row of conical processes, extending along the posterior 



' A similar condition of the circtilatory system has been observed by Nordmann, 

 (Jiiatrefages, Van Beneden, and AUmann, in various Nudibranchiata, though perhaps not 

 quite so distinctly. 



