196 Dr. T. Williams on the Mechanism of Aquatic 



as one current at the posterior extremity, but from above down- 

 wards, from the dorsal to the ventral side, in as many streams 

 as there are spaces (PI. XV. fig. 1 e) between the secondary lobuli 

 of the organs. By this arrangement the inspiratory pure current 

 is first and at once brought into contact with that half of the 

 gill on which the vascular laminae are suspended. It effects its 

 escape, during the systole of the organ, at the ventral side, 

 through the fissures between the 7^o^^-vascular supports of the 

 lobuli [dj d). Although in the genus Loligo the gill (fig. 3, 3^) 

 does not fold so much upon itself as to enclose a cylindrical 

 axis, as in Octopus (fig. 1), the water-currents observe pre- 

 cisely the same direction in both. The admission of the aerating 

 element apparently into the interior of the gill in the Cepha- 

 lopods may be said, on the one hand, to be parallel to what 

 occurs in the Lamellibranchiata, or, on the other, may be 

 likened to the entrance of the air, in the Mammal, into the 

 lungs. As a respiratory mechanism, it resembles the latter 

 more than the former. The lungs possess the property of di- 

 lating and contracting, in order to inhale and exhale the gases 

 concerned in respiration. The branchiae of the Cephalopod are 

 endowed with the same property. They bear a nearer similarity 

 to the gills of the Pectinibranchiata than to those of any other 

 Mollusks. They are highly muscular and irritable : the disposi- 

 tion of the muscles will be afterwards described. On the floor 

 of the branchial chamber, situated immediately underneath 

 each gill, and running parallel with these organs, may be ob- 

 served a dense prominent ridge (PI. XV. fig. 1 h, h), to which 

 the branchiae are attached, and upon which they rest. This 

 ridge consists of a dense bundle of muscles, which during con- 

 traction are capable of approximating closely together the two 

 extremities of the branchiae — in other words, of shortening these 

 organs in length. These muscles are richly supplied with nerves 

 from the neighbouring paUial ganglion. They aid in a very 

 material manner the branchial movements of dilatation and con- 

 traction. They not only afford a fixed point of attachment to 

 the gills, but they are accessory to the mantle in the respiratory 

 movements. 



Structure of the Branchice. , 



The Tetrabranchiate and Dibranchiate orders are founded 

 simply upon the number of the gills*. No classification has 

 been attempted on the basis of the remarkable varieties which 

 occur in the anatomical structure of the gills. As far as the 



* The second pair of branchiae in the Tetrabranchiate Cephalopods are 

 most probably parallel to, and representative of, the supplementary gills of 

 the Lamellibranchiata described m a former paper. 



