Monograph of the LamcUihrancliiitta. 213 



they are connected by a ligament, -within which is an elastic substance, 

 termed cartilage, which forces the valves open ; the valves are closed 

 in the lower groups by one central adductor muscle, and in the higher 

 types by two or more. Respiration (as the name implies) is effected 

 by lamellar branchire, or gills, which are four, crescent shaped, and 

 attached to the inside of the mantle ; one pair on each side of the 

 visceral mass. Each valve is lined by a lobe of the mantle, and the 

 attachments of the muscular fibers, for retracting its edges,' form the 

 pallial scar in the shell ; in the low types the lobes are open and dis- 

 connected, as in the Palliohranchs, and there is little or no foot ; in the 

 higher types the edges of the two lobes are united posteriorly, and 

 produced into two tubular siphons ; one branchial, through Avhich the 

 water enters to the gills, and carrying nutrient particles to the mouth ; 

 the other anal, through which the impure water constantly flows out, 

 the currents in each case jjroduced by vibratile cilire ; the attachments 

 of the line of muscular fibers for retracting the siphon form the 

 "pallial sinus" in the shell of such as have the siphons. The animal 

 is without head or cephalic organs of sense ; the mouth is at the 

 anterior end, without organs of mastication, having two strap-shaped, 

 sensitive tentacles on each side, the upper pair of which correspond, on 

 a reduced scale, with the spiral arms of the Brachiopoda ; the lower 

 pair (which equal the upper in size)' are analogous to the transverse 

 brachial, fringed part below the mouth in the same ; a short oesophagus 

 leads into a pear-shaped stomach, leading to a convoluted intestine, 

 the straight terminal portion, or rectum, perforating the heart and 

 terminating at the base of the anal siphon, or over the posterior 

 adductor. The liver is a large follicular mass, enveloping the stomach ; 

 a glandular mass, partially surrounding the rectum, is called kidney, 

 by Owen, from uric acid having been found in it, but is marked ovary 

 by Milne Edwards (both agree that the ovary in the female and testis 

 in the male surrounds the intestine). The foot, when it exists, is a 

 symmetrical, fleshy, extensile organ, developed from the ventral 

 aspect of the body ; the enveloping muscular fibers are transverse, tlie 

 action of which protrudes the foot, and longitudinal, the action of 

 which retracts it. In the lower bivalves (jnonomyaria), the heart is of 

 a single auricle, and one ventricle, perforated by the rectum ; in the 

 higher groujxs (Dhnyaria), the veins from the gills open into two 

 auricles, which open into one fusiform ventricle, perforated by the 

 intestine (in Area the ventricle is double). An artery from each end 

 of the ventricle distributes the blood. Nerves — in the monomyaria the 

 principal ganglion is on the ventral convex edge of the adductor, to 

 which it sends branches, as well as to the gills and mantle ; two 



