58 



Guide to the Invertehrata. 



GALLERY the mouth currents of water containing food-particles ; it is called 

 East Side, the " lophophore " (tuft-bearer), and resembles the structure of 

 Wall-cases the same name in the Bryozoa (Diagram 8). This lophophore 

 T^w ^^' ^^ often produced into two processes, which fill the forepart 

 cases and sides of the shell-cavity and are often spirally coiled. These 



86-88. processes are generally known as '* arms," and, since they were 

 formerly supposed to represent the "foot" of the Mollusca, their 

 presence suggested the name BfiAcniopoDA {^paxii^v, an arm ; 

 and TToi's, a foot). The arms are often supported by a calcareous 

 skeleton, the shape of which is of great importance in classifying 

 fossil Brachiopods. Thus, the Telotremata branched into loop- 

 bearing forms (Diagram 9 and Fig. 102, Magellania) and spire- 

 bearing forms (Diagram 9, Atrypa^ and Fig. 101, Spirifer). 



The mouth leads to a slightly coiled intestine (Diagram 10), 

 which in the Inarticulata is provided with an anus (whence this 

 class has also had the name Tretenterata ' applied to it). It seems 

 that some of the earlier Protremata were also provided with an 

 anus ; but in the later Articulata this structure became degenerate, 

 and no longer exists in the living representatives of the class ; 

 for them, therefore, the name Clistenterata * was proposed. 



The Brachiopoda possess a system of blood-vessels with a con- 

 tractile heart, a distinct nervous. system, and a pair of excretory 

 organs (''nephridia "), which serve also for the transmission of 

 the generative products (Diagram 11). The sexes are usually 

 separate. 



The Brachiopoda are found in seas all over the world, and 

 usually at depths of less than 100 fathoms, but they have been 

 dredged at a depth of 2,900 fathoms. Most attach themselves 

 permanently to a hard bottom by the peduncle, open their shell 

 so far as the hinge permits, and collect miimte food-particles in 

 the currents of water that flow down the lophophore. Some 

 protrude and even unroll the arms. Lingula, as shown in 

 Diagram 2, lives in a tube in the sand, forming a case of 

 agglutinated sand around the lower end of its peduncle; it 

 stretches its shell to the opening of the tube, and the projecting 

 setae guide the currents of water down to the lophophore; but 



^ Meaning "pierced guts.' 

 ' Meaning "closed guts." 



