BRIEF REVIEW OF THE SYSTEM OF LIFE. 



425 



two (1, 2, Figs. 396, 397), rarely more, adductor muscles, and also an impression of the 

 mantle or pallium, which is concentric with the lower and hinder margin of the shell 

 in integripallial species, and has a sinus posteriorly in sinupallial species. Tlie mantle 

 is large, concealing the body, with the two sides either free at the lower edge, or not con- 



393-401. 



393. 



MOLLUSKS, Figs. 393-401. — (1) Brachiopods : 393, Terebratula irapressa, of the Oolyte; 394, Lingula on 

 its stem. (2) Bryozoans : 395 (x 8), 395 a, genus Eschara. (3) Lamellihranchs : 396, 397, 398, the 

 Oyster. (4) Gastropods : 399, Helix. (5) Pteropods : 400, genus Cleodora. (6) Cephalopods : 401, 

 Nautilus (x J). 



nected (as in the Oyster, etc.), or else grown together into a sac (Venus, Mya); and in 

 the latter case usually having the sac terminate behind in two tubes, as in 3Iya, Solen, 

 one incurrent, for receiving water, to the gills, and food, and the other excurrent. Imper- 

 fect eyes or eye-spots exist in the mantle of some species. Gills are usually lamellar 

 organs (whence the name, Lamellibranchs) situated between the mantle and the body. 

 In a few boring species, the shell includes, or is followed by, a long, calcareous tube, 

 which may be 1 to 2 feet long in Teredo, the timber-borer. 



4. MoUuscoids. 



I. Brachiopods. — Brachiopods (Figs. 393, 394, and 402-430) have a bivalve shell, 

 and m this respect are like the Lamellihranchs or ordinary bivalves. But the shell, 

 instead of covering the right and left sides, covers the dorsal and ventral sides. More- 

 over, it is symmetrical in form, and equal, either side of a vertical line ab, Fig. 407. The 

 valves, moreover, are almost alv-ays unequal ; the larger is the ventral, and the other the 

 dorsal. There is often an aperture at the beak (near 6, Fig. 393), that in the young state 

 and often through the adult gives exit to the pedicel, by means of which the animal 

 is fixed to some support. Species having the two valves hinged together are called Articu- 

 late Brachiopods, and those that are hingeless are the Inarticulate. Some of the genera 

 of the former group are Orthis, Ovthisina, Spirifer, Bhynchonella, Strophomena, Penta- 

 merus, Terebratula; and some of those of the latter are Lingula, Lingulella, Obolus, 

 Obolella, Discina, Crania. 



Brachiopods have a pallium, but no independent branchial leaflets ; and a pair of 

 coiled fringed arms, which in some cases may be extruded, — whence the name Brachio- 

 pod, meaning arm-like foot. For the support of these arms, there are often bony 

 processes (Figs. 402, 406, and 409). These calcified arm-supports, when present, are 2 

 thin lamella;, attached to the interior of the dorsal valve ; they are short and curved in the 



