DEVELOPMENT OF BBAOHIOPOBS. 193 



creases much in size so as to partially enclose the cephalic 

 segment, as at C. The form of the Brachiopod is then soon 

 attained, as seen in D, in which the head (c) is seen project- 

 ing from the two valves of the shell {tli), the larger being 

 the ventral plate. 



The hinge margin is broad and slightly rounded when 

 looked at from above ; a side view, however, presents a wide 

 and flattened area, as is shown in some species of Sjnrifer, 

 and the embryo for a long time takes the position that the 

 Spirifer must have assumed (Morse). Before the folds have 

 closed over the head, four bundles of bristles appear ; these 

 bristles are delicately barbed like those of larval worms. 

 The arms, or cirri, now bud out as two prominences, one on 

 each side of the mouth. Then as the embryo advances in 

 growth the outlines remind one of a Leptwna, an ancient 

 genus of Brachiopods, and in a later stage the form becomes 

 quite unlike any adult Brachiopod known. 



The deciduous bristles are then discarded, and the perma- 

 nent ones make their appearance, two pairs of arms arise, 

 and now the shell in " its general contour recalls Sijjlwno- 

 treta, placed in the family DiscinidoB by Davidson, a genus 

 not occurring above the Silui-ian." No eye-spots could be 

 seen in Terebratuhna, though in the young Thecidium they 

 were observed by Lacaze-Duthiers. The young Terebratu- 

 hna differs from Discina of the same age in being sedentary, 

 while, as obseived by Fritz Muller, the latter "swims freely 

 in the water some time after the dorsal and ventral plates, 

 cirri, mouth, oesophagus and stomach have made their ap- 

 pearance." Discina also differs from Terebratuhna in hav- 

 ing a long and extensible CBSophagus and head bearing a 

 crown of eight cirri or tentacles. Kegarding the relations 

 of the Brachiopods with the Polyzoa, Morse suggests that 

 there is some likeness between the young Brachiopod and 

 the free larva of Pedicellina. Fig. 133, B. represents the 

 Tercbratuliua when in its form it recalls Megerlia or Argi- 

 ope. C represents a later Lingula-like stage. "It also 

 suggests," says Morse, "in its movements, the nervously 

 acting Pedicellina. In this and the several succeeding 

 stages, the mouth points directly backward (forward of 



