626 LECTURE XXIV. 



returns the blood to a single median systemic heart, which is rounded 

 in the Octopods, lozenge-shaped in the Calamaries, and fusiform, but 

 bent upon itself, in the Sepia {ni) Sepioteuthis, and Sepiola. It 

 sends oiF two aortae, as in the Nautilus, the larger or posterior one 

 (r) 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 Vertebrate animal ; 

 but it supplies the head and all its complex radiating appendages. 

 Dilatations of the branchial arteries, like accessory hearts, have been 

 observed * in the substance of the arms of the great Calamary {Loligo 

 sagittata). Hugh Miller, in a vivisection of this species, saw " the 

 yellow tubular heart propelling into transparent tubular arteries 

 yellow blood," and he noticed lying detached from it " the two deeper- 

 coloured hearts." f The small aorta supplies the stomach, intestine, 

 branchiae, and genital organs. 



The branchiae {Jig. 227, t.fig. 228, i) are two in number, as the 

 name of the order indicates. They are concealed, as in the Nau- 

 tilus, by the mantle, f, which extends in front of the viscera to 

 form the branchial chamber : the muscular tube called the funnel 

 {fig. 227, i) projects from its outlet. The rectum opens 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 Mollusca generally. Each 

 gill consists, as in the Nautilus, of a number of triangular vascular 

 laminaa, extending transversely from either side of the fleshy glandular 

 stem, and decreasing in size to the extremity of the gill : each plate 

 is composed of smaller transverse laminae, which are themselves simi- 

 larly subdivided, the entire gill presenting the tripinnate structure, 

 which aifords an extensive, though close-packed, surface for the 

 minute subdivision of the blood-vessels. In Loligopsis each gill has 

 twenty-four pairs of plates : in Sepia, thirty-six pairs ; in 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 branchial artery extends along the attached 

 border or stem, the branchial vein along the free border : the circu- 

 lation through the artery is accelerated by the contraction of the 

 surrounding fibres of the stem. 



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 Mollusks. The 



♦ CCCCVI. t CCCXCIV. p. 447. 



