194 



INVERTEBRATE ANIMALS. 



The inner surface of the valves of the shell is lined by expansions 

 of the integument, -which are called the " mantle-lobes," and which 

 secrete the shell. The digestive organs and muscles occupy a small 

 space near the apex or " beak " of the shell, "which is partitioned off 

 by a membranous jiartition, perforated by the aperture of the mouth. 

 The remainder of the cavity of the shell is almost filled by two long 

 processes, derived from the sides of the mouth, fringed with lateral 



Fig. 135. — Terebratula {W«ldheimia) fiavescens. A, The sliell vie-wed from behind, 

 .showing the dorsal valve, and the perforated summit of the ventral valve above it. 

 B, Inner view of the dorsal valve, showing the shelly loop (0 whicli supports the 

 spiral arms. C, Inner view of tlie ventral valve, showing tlie foramen or aperture 

 (/) in the beali, tlirough wliich the muscular stallt of attachment passes. D, 

 Longitudinal and \eriical section of the animal, showing the spiral arms (o), the 

 stomach (3), and tlif liver (h). At / is tiie opening in the beak, with the sialic of 

 attacliuient (p) passing tlirnugli it. (Afti-r Davidson and Owen.) Some details 

 have been omitted in li;4s. B, C, and D, for tlic sske of clcarnf ss. 



branches, and termeil the "arms" (fig. 135, D"). These arms are 

 usually closely coiled up, and serve to obtain food for the animal. 

 It is from these organs that the name of the class is derived (Gr. 

 brac/iion, arm ; and jmde/t, feet). The arms also serve as respiratory 

 organs, and in many forms they are supported on an internal calca- 

 reous framework fir skeleton, sometimes called the "carriage-spring 

 apparatus.'' 



