PTEROPODA. 427 



upper surface of which is seen, when highly magnified, to be cover- 

 ed with regular rows of spiny booklets, all directed backwards, and 

 evidently intended to assist in deglutition (Jig. 199, B). 



The structure of the alimentary canal is extremely simple. The 

 oesophagus (Jig. 200, t) gradually dilates into a wide stomachal 

 cavity that is surrounded on all sides by the mass of the liver ; 

 while the intestine (u), in which the stomach terminates, mounting 

 towards the left side of the neck, ends by an external anal orifice. 

 Two long and slender salivary glands (w) are placed at the sides 

 of the oesophagus, and furnish a secretion that is poured into the 

 mouth. The precise character of the bile-ducts has not been satis- 

 factorily determined in Clio ; but in Pneumodermon, another 

 Pteropod very nearly allied to the genus we are describing, the 

 stomach itself, which is enveloped on all sides by the liver, receives 

 the biliary secretion through a multitude of minute pores. 



(467.) With respect to the real nature of the respiratory appa- 

 ratus in C/zo, much doubt exists. Cuvier regarded the aliform fins 

 as being subservient to respiration, as well as forming locomotive 

 organs ; and observes, that the surfaces of these appendages seen 

 with the microscope present a network of vessels so regular, so close, 

 and so delicate, that it is not possible to doubt but that they are 

 intended to perform the functions of a respiratory apparatus, and 

 states, moreover, that their connection with the internal vessels and 

 the heart confirms this view of the nature of these membranes. 



Eschricht, on the contrary, denies altogether the existence of any 

 such vascular ramifications as Cuvier describes ; asserting that the 

 appearance alluded to is entirely produced by the spreading out of 

 the muscular fibres above mentioned, and that the only vessels 

 visible in the alar processes are a few arterial branches derived from 

 the aorta. 



We are still, therefore, in ignorance as to the respiratory organs 

 of Clio ; the heart, however, is very apparent : it is composed of a 

 single auricle and ventricle, enclosed in a pericardium (Jig. 200, 

 m), and gives off at one extremity a large vessel (w), which Cuvier 

 regarded as a pulmonary vein, but which Eschricht has proved to 

 be the aorta, inasmuch as he has traced its branches to the liver 

 and the other internal viscera of the body. 



(468.) The nervous system of this mollusk is easily distinguish- 

 ed, not only on account of the large proportionate size of the ganglia, 

 but from the circumstance of the nerves being of a pale red colour. 

 The ganglia form a ring (Jig. 200, y) placed around the cesopha- 



