THE BRANCHIOGASTEROPODA. 35 



the foot and the mantle lobes in front, and between the mantle 

 lobes posteriorly (Fig. 1 3). The branchial organs may consist of 

 distinct filaments, or of plates composed of tubular rods support- 

 ing a network of blood-vessels, and covered with cilia, by the 

 action of which they are constantly bathed by currents of water. 



The nervous system presents a no less distinct advance than 

 the other organs. All Lamellibranchs possess at least three 

 pairs of principal ganglia — a cerebral pair at the sides of the 

 mouth, a pedal pair in the foot, and a third pair on the under 

 surface of the posterior adductor muscle, which are commonly 

 called " branchial," but which, as they supply not only branchial, 

 but visceral and pallial filaments, may more properly be termed 

 " parieto-splanchnic." Three sets of commissural filaments con- 

 nect the cerebral ganglia with one another, with the pedal, and 

 with the parieto-splanchnic ganglia. The inter-cerebral commis- 

 sures surround the mouth, and the other two pairs of cords 

 extend respectively, from the cerebral to the pedal, and from 

 the cerebral to the parieto-splanchnic ganglia. 



Finally, there is always, in these animals, an external shell, 

 which is formed as an excretion from the surface of the lobes of 

 the mantle, and is composed of layers of animal matter hardened 

 by deposit of carbonate of lime, which may or may not take a 

 definite form, and so give rise to " prismatic ' : and " nacreous " 

 substance. As the lobes are right and left, so the valves of the 

 shell are right and left, and differ altogether from the valves of 

 the shell of the Brachiopoda, which are anterior and posterior. 

 The valves of the shell can be brought together by adductor 

 muscles. Of these one (Fig. 12, II) always exists, posteriorly,- 

 on the neural side of the intestine. A second (Fig. 12, I) is 

 commonly found anteriorly to the mouth, on the haemal side of 

 the intestine. 



The hiatus between the next class, which is termed Bkan- 

 chiogasteropoda in the table, and that just defined, is consider- 

 able, though not quite so well marked as that between the 

 Ascidians and the Lamellibr and data. This group, which con- 

 tains the whelks, periwinkles, sea-slugs, and the Heteropoda of 

 Cuvier, consists of animals which, like the Lamellibranchs, pos- 

 sess (in their young state, at any rate) a mantle ; a foot, which 



d 2 



