CH. XV] 



FORM OF SHELL 



371 



to a rock so that when it has once fixed itself a Brachiopod cannot 

 change its place of residence 

 (Fig. 176). Each valve of 

 the shell is lined by the body- 

 wall of the animal, but the 

 body does not occupy the 

 whole of the space between 

 the two valves ; it is produced 

 into two folds or flaps called 

 the mantle -folds, which are 



FIG. 173. Terebratula semiglobosa, a fossil 

 Brachiopod shell x f . 



A. Dorsal view. B. Lateral view, 



a. posterior, b. anterior eud. The line 

 between a and b is called the length, it 

 traverses the aperture through which 

 the stalk projects. The line between 

 c and d is the breadth, between e and 

 f the thickness, and between g and h 

 the hinge-line. 



each hollow and contain ex- 

 tensions of the coelom. These 

 secrete the larger part of 

 the valves of the shell. In 

 Lingula and some others the 

 free edges of these man tie- 

 folds, lying parallel with the 

 free edges of the shells, bear 



a number of chaetae which recall those of the Chaetopoda. It is 

 by no means certain that the shell of Brachiopod s is an external 

 secretion like that of Mollusca : it seems possible that it is really 

 deposited in the connective tissue under the ectoderm. 



In most of the thick-shelled forms 

 the shell is traversed by processes of 

 the mantle, which nourish it, so that 

 in dried Brachiopods the shell seems 

 perforated with a number of pores. 



If we open the valves of the shell 

 of a living Brachiopod (slightly so as 

 to avoid tearing the tissues) and look 

 in, we shall see between the ventral 

 and dorsal mantle-folds the anterior 

 body-wall of the creature. This some- 

 times runs almost horizontally across 

 between the space within the valves, 

 but often slopes obliquely from the 

 ventral to the dorsal valve of the shell. Part of this wall is modified 

 to form two long ridges, the ends of which project freely and are called 

 the arms; they are coiled and are beset with tentacles (Fig. 175), 

 Running close to the origin of the tentacles is a little lip or flange 

 so placed that between it and the tentacle is a groove or gutter. The 



242 



Fia. 174. A section through the 

 shell of Waldheimia flavescens. 

 Magnified. 



a. Prismatic layer formed in con- 

 nective tissue. 6. Epidermal 

 layer. c. Outer calcareous 

 layer. d, e. The expanded 

 outer ends of the tubes which 

 traverse the shell. 



