DEVELOPMENT OF THE BRACJIWPODA 251 



enclose the uiiterior end (figures 108-111). The surfaces of 

 the mantle which were exterior in the cephalula have now 

 become inner and the bundles of setse have revolved 180°, 

 changing their direction from posterior to anterior. This 

 leaves the lower part of the thoracic, and the whole of the 

 caudal, segment exposed. The outer surface of the mantle is 

 invested w4tli a hard integument, which, upon completion 

 and before the growth of the true shell, forms the protegu- 

 lum. The pedicle at this stage is also defined, being a modi- 

 fication of the caudal segment. It may serve to attach the 

 larva to foreign objects, as in Cistella (figure 94) and Tere- 

 brafuUna, or it may remain undeveloped for a time, as in 

 Glottidia and Discinisca. A rudimentary digestive tract is 

 present. 



The body muscles which have been developed thus far 

 consist of four distinct pairs. Two pairs lie close to the 

 sides of the body cavity, and extend to the points of inser- 

 tion of the bundles of bristles (figure 94, ad}. They become 

 after transformation the four adductor muscles of the valves. 

 The third pair extends from the ventral side of the body to 

 the caudal segment, and is converted into the ventral pedicle 

 muscles (figures 94, 99, 100, vp). The fourth pair is situated 

 posterior to the digestive tract, and extends from the dorsal 

 to the ventral wall of the body (figure 94, di). They form 

 the divaricator muscles in the mature brachiopod (figure 100, 

 di}, and are divided into or duplicated by a pair of dorsal and 

 a pair of ventral divaricators. There is also a pair of dorsal 

 pedicle muscles in the larva of Liothyrina and Terebratulina. 



The folding upward of the mantle lobes forms the first 

 hinge-line of the future valves (hi, figures 110, 111). Thus 

 its origin is not, as in pelecypods, a line produced by the 

 bending of a single plate (Jackson), but is the line along 

 which the two mantle lobes are bent against the body. 

 Between them projects posteriorly nearly half the body of 

 the animal, and the whole opening corresponds to the j)edicle- 

 opening of later stages of growth. The hinge of brachiopods, 



