CEPHALOPODA. 357 



Sepiola. It sends off two aortae, as in the Nautilus, the larger or 

 posterior aorta being provided with a muscular bulb, and with two 

 semicircular valves at its origin. The distribution of the large aorta 

 very closely resembles that of the Nautilus. The vascular ring which 

 encircles the oesophagus typifies the branchial arches in the Verte- 

 brate animal ; but it supplies the head and all its complex radiating 

 appendages. 



The branchiae {Jig. 136. t. Jig. 137. i) are two in number, as the name 

 of the order indicates. They are concealed, as in the Nautilus, by 

 the mantle, which extends in front of the other viscera to form the 

 branchial chamber: a distinct muscular tube projects from its out- 

 let. The rectum and the generative organs open into the branchial 

 chamber at the base of the funnel, manifesting the same relation of 

 the breathing organs to the termination of the alimentary canal, which 

 characterises the lower orders of the Mollusca. Each gill consists, 

 as in the Nautilus, of a number of triangular vascular laminae, ex- 

 tending transversely from either side of the fleshy stem, and de- 

 creasing in size to the extremity of the gill; each plate is composed 

 of smaller transverse laminag, which are themselves similarly sub- 

 divided, the entire gill presenting the tripinnate structure, which af- 

 fords the most extensive surface for the minute subdivision of the 

 blood-vessels. In the LoUgopsis each gill has twenty-four pairs of 

 plates: m the Sepia, thirty-six pairs ; in the Loligo sagittata, sixty 

 pairs. The stem of the gill is not only attached by its base, but by a 

 thin fibrous membrane through nearly its whole length to the mantle. 



The mechanical part of the respiratory act is performed by the 

 muscular actions of the mantle and funnel, the gills not being pro- 

 vided with vibratile cilia, as in many of the inferior Mollusca. The 

 w^ater is admitted into the branchial cavity at the anterior aperture of 

 the mantle, outside the base of the funnel. Two large valvular folds 

 of fibrous membrane, which are concave towards the respiratory 

 cavity, prevent the currents from escaping by this entry : they are, 

 therefore, propelled by the whole force of the contraction of the 

 muscular mouth through the cavity of the funnel, the base of which 

 is articulated, in most of the Cephalopods, by lateral joints, with the 

 sides of the anterior aperture of the mantle. 



At first sight it might seem that the Cephalopods which had bui 

 two gills were less efficiently provided with the means of respiration 

 than those which have four ; but increased number, irrespective of 

 correlative structure in an organ of the animal body, is ever a mark 

 of its inferiority; and the four branchiae of the Nautilus forcibly 

 illustrate the true character of vegetative repetition, when contrasted 

 with the two gills of the cuttle-fish in connection with their super^ 



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