447 



nences, one on each side of the mouth. Then as the embryo ad- 

 vances in growth the outlines remind one of a Leptaena, 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 permanent 

 ones make their appearance, two pairs of arms arise, and now the 

 shell in " its general contour recalls Siphonotreta, placed in the 

 family Discinidae by Davidson, a genus not occurring above the 

 Silurian." No eye spots could be seen in Terebratulina, though 

 in the young Thecidium they were observed by Lacaze-Duthiers. 

 The young Terebratulina differs from Discina of the same age in 

 being sedentary, while, as observed by Fritz Midler, the latter 

 " swims freely in the water some time after the dorsal and ventral 

 plates, cirri, mouth, oesophagus and stomach have made their 

 appearance." Discina also differs from Terebratulina in having a 

 long and extensible oesophagus and head bearing a crown of eight 

 cirri or tentacles. Regarding the relations of the Brachiopods 

 with the Polyzoa, Morse suggests that there is some likeness be- 

 tween the embryo Brachiopod, and the free embryo of Pedicellina. 

 Fig. 192, B, represents the Terebratulina when in its form it re- 

 calls Megerlia or Argiope. 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 authors), or away 



Fig. 192. 



from the perpendicular end (D) and is surrounded by a few cili- 

 ated cirri, which forcibly recall certain Polyzoa. The stomach and 

 intestine form a simple chamber, alternating in their contractions 

 and forcing the particles of food from one portion to the other." 

 Figure 192, E, shows a more advanced stage, in which a fold is 

 seen on each side of the stomach ; from the fold is developed the 



