INSECTS. BRACHIOPODA. 



109 



valve is symmetrical in itself, its right and left halves re- Gallery 

 sembling one another. On the other hand, one valve is nearly VIII. 

 always larger than the other (Figs. 55 a and 60 a). By the 

 larger valve the adult animal is usually attached to rocks or 

 other objects. In a few forms, such as Crania (Diagram 1), 

 the valve is directly cemented to the rock by its own sub- 

 stance. In others, such as Lingula (Diagram 2 and Fig. 57), 

 the valve is attached by a long muscular stalk, the " peduncle " 

 or ''pedicle," which is capable of waving movement and of 

 contraction. There are also intermediate stages, with shorter 

 peduncles, such as Heniithyris (Diagram 8) and Orhimloidea 

 (Fig. 58 h). This larger, attached valve is therefore often called 

 the " peduncle valve " : by English writers it is called the 

 ventral valve," although in the natural position of stalked 

 forms it is always the uppermost. Similarly the smaller 

 valve is called the " dorsal valve " ; but a better name is 

 "brachial valve," which reminds one that this valve often 

 bears a calcareous skeleton supporting the so-called " arms " 

 (Diagram 9 and Figs. 59, 60). 



In microscopic structure also the shell differs from that 

 of the Mollusca (Diagram 3). It is mainly composed of 

 small prisms of calcite (carbonate of lime), which usually lie 

 at right angles to the surface of the shell. In the harder- 

 shelled forms these make up the greater part of the shell, 

 merely being coated on the surface with a layer of slightly 

 different texture and with a thin horny membrane. In the 

 softer-shelled forms, such as Lingula, horny substance occurs 

 not merely on the surface, but in layers Ijetween the prisms, 

 which in this case are of phosphate of lime. In many genera, 

 such as Terebratella, Crania, Cistella, the shell is perforated 

 by a number of small canals ; these contain processes of the 

 mantle, the arrangement of which is shown in Diagram 3a. 

 In fossils this structure gives to the exterior of the shell a 

 pitted or " punctate " appearance under a magnifying glass, 

 and thus enables one to distinguish such shells from those 

 which are " impunctate," as the shells of Atrypidse and most 

 Ehynclionellidge. 



The shell-valves are secreted by the two mantle-folds 

 which line them. These are extensions of the body -walls, 

 and they contain prolongations of the body-cavity, in which 

 is a blood-like fluid and in which the generative products 

 are formed. These vessels often produce impressions on 

 the inside of the shell, and so can be traced in the fossils, Wall-case 

 e.g. Camarojphoria (Fig. 56) and a fine specimen of Orthis 



