ORGANS OF RESPIRATION^ ETC. 35 



movement, whilst in the decapods they are free and capable of 

 movement : in the former the skin is susceptible of contraction 

 so as to cover the eye entirely, fulfilling the functions of an eye- 

 lid ; whilst in the latter the littoral species are furnished with 

 this protection, but the pelagic ones are without it. 



B. Auditory Organs In the dibranchiates the auditory sacks 

 are lodged in cavities of the cephalic cartilage : the}'' each con- 

 tain a single, large, calcareous otolite. In the Nautilus, however, 

 these sacks are found attached to the pedal ganglia, and contain 

 numerous otolites. The external ears are hollow, plicated pro- 

 cesses on the side of the eyes, communicating through a passage 

 lined by a glandular membrane, with the auditory sacks. 



C. 01 factor t/ Ori'/aiis. Kolliker has made the interesting dis- 

 coveiy that a pair of pits or papillae, as the case may be, situated 

 behind or above the eye, are olfactory organs. They are pits 

 above the eyes in the Teuthidse and Sepiadae and in some of the 

 Octopods, but in Argonauta and Tremoctopus they are devel- 

 oped as papilla?, and in Nautilus are elongated like small tentacles, 

 placed immediately behind the 63 7 es. D'Orbigny has mistaken 

 them for external ears. 



Organs of Respiration , Circulation and Secretion. 



The heart, which is placed on the haemal side of the intestine, 

 receives the blood through contractile vessels connecting it with, 

 and equal in number to the branchiae ; these may be regarded as 

 auricles. The branchia- are not ciliated, and are generally if not 

 always themselves contractile. The arteries end in an exten- 

 sively developed capillary system, but the venous channels retain 

 to some extent the character of sinuses. In returning to the 

 heart, the venous blood is oathi-red into the vena cava, a large 

 longitudinal sinus, which is situated on the posterior side of the 

 body close to the anterior wall of the branchial chamber, and 

 divides into a number of branchial vessels corresponding with 

 the number of branchiae. Each of these vessels traverses a 

 chamber in communication with the mantle cavity (and which 

 may be considered a renal organ), and that portion which comes 



