CLASs II BRACHIOPODA 361 



Brachial supports. — Of special systematic importance are the brachidia, or 

 internal skeleton of the fleshy arms (Fig. 531), which occur in the Spiriferacea 

 and Terebratulacea. The brachidia are, as a rule, prolonged basally from the 

 crura, and are extremely variable in form. They usually pass through a 

 more or less complex series of metamorphoses during the growth of the 

 individual, and do not attain their complete development until the animal has 

 reached maturity. 



The simplest form of brachial supports is found in the Rhynchonellacea and 

 Pentameracea, where it consists of two short, or only moderately long, curved 

 processes called the crura when discrete, and cruralium when the plates are 

 united. The crura are attached to the hinge-plates. The cruralium is 

 formed by the union ;of the crural plates in the Pentameracea. It serves 

 for the attachment of muscles, and may either rest upon the bottom of the 

 valve, or may be supported by a median septum. When the crura remain 

 separate, and are therefore not for muscular insertion, they are homologous 

 with and the equivalent of the crura in the Rhynchonellidae. 



In the Spiriferacea, two thin, spirally coiled ribbons, or spiralia, are 

 given off from the crura ; the coils exhibiting great diversity in form, in the 

 number of volutions, and in the direction of the hollow cones (Fig. 531, 

 B, C, D). The spiralia are usually joined by a transverse band or jugum 

 (Fig. 531, D). When the latter is discontinuous, the parts are called the 

 jugal processes. The bifurcations of the jugum may enter between the con- 

 volutions of the spiralia, and may be continuous with them to their outer 

 ends, forming what is termed a double spiral or diplospire (Fig. 531, 5). In 

 the Terebratulidae, the brachia are also extensions of the crura, and form free, 

 shorter, or longer loops, which depend toward the anterior margin (Fig. 531, 

 E, F). The two descending hranches may either unite directly or may be 

 joined by a transverse band ; or the descending branches may recurve, continue 

 upward as ascending branches, and be connected posteriorly by a transverse 

 band. In the Terebratellidae, during all or some portion of the animal's 

 existence, the loops are attached to a median septum by outgrowths from the 

 descending lamellae (Fig. 531, 6^). In the Stringocephalidae and Megathyrinae 

 (Fig. 531, H) the descending branches are parallel to the lateral margins of 

 the Shell, and unite along the median line ; but in some degenerate species 

 a remnant of the loop is represented by a transverse band situated centrally 

 on the median septum. The entire form of the brachidia is manifestly 

 dependent upon the character of the convolutions of the fleshy arms. In 

 recent Hemithyris (Fig. 531, A) the brachia form hollow spiral cones, and if 

 we imagine these as supported by a calcareous framework, the result will be 

 a form of support like that seen in the Atrypidae. The fleshy arms of the 

 Terebratellidae are continuous with, and have at first the form of the loop, 

 but later develop a coiled median arm. Here the loops only have calcareous 

 supports ; but in the Spiriferacea the entire brachia are provided with an 

 internal calcareous skeleton. 



The changes in the form of the brachidia in the Telotremata during the 

 ontogenetic stages of the individual furnish very important data in regard 

 to the relationships existing between the different groups. In the Spiri- 

 feracea, not only does the number of convolutions of the spirals increase with 

 age, but the brachidia begin with Centronella- and Dielasma-\\\Q loops, from 

 the outer ends of which the spires are developed. Still more striking are 



