Report of the State Geologist. 



495 



The Man ii.k. 

 This is a thin transparent membrane, appearing in the embry 

 onic condition of the animal, as t wo distinct Lobes. Ii is, primarily, 



the shell-secreting gland of the animal, and in all periods of 

 growth it lines the entire inner surface of the shell. 



In Ckania, it is composed of three layers, a middle one, of 

 compact, non-celluliferous tissue or cartilage, on each surface of 



Fig. 100.— The animal of Magellania australis, viewed from the dorsal side. 

 a, mantle ;£&, body: c, pedicle; d, trunks of pallial sinuses; e, branches of pallial sinuses; J, 

 genital organs; g, posterior adductor muscles; h, marginal fold of mantle; i, setse; j, circum- 

 pallial vessel; k, edge of mantle; I, median fissure corresponding to septum of valve; m, depres- 

 sion caused by hinge-plate; p, anterior adductor muscles; r, diductors; s, accessory diductors; 

 t, liver.— (Hancock.) 



which is a single layer of cells. From that which lies against 

 the surface of the shell arise the caeca or blind tubes which enter 

 the perforations of the test. At various points the mantle, or 

 certain of its layers, folds upon itself, inclosing cavities or pallial 

 sinuses which contain the circulating fluids and often portions of 

 the genital organs. These sinuses frequently modify the interior 



47 



