DEVELOPMENT OF THE BRACHIOPODA 277 



as shown by all the other leading characters. But when this 

 classification of arm structures is applied within a family or 

 genus, or even when made the basis of comparison among some 

 closely related families, it is sometimes possible to reach very 

 satisfactory conclusions relating to the systematic position of 

 various forms. 



Leiolophus Stage. 



It is hardly necessary to direct attention to the embryonic 

 brachial structure before the growth of any of the tentacles, 

 or cirri, on the edge of the lophophore, while the animal is in 

 the typembryonic stage. For the sake of designating all the 

 stages, this may be called the leiolophus stage, though it has 

 no special significance beyond indicating the beginning of the 

 lophophore. 



Taxolophus Stage. 



The first stage in which a true brachial structure is mani- 

 fest is an early larval form, often the protegulum stage, when 

 the tentacular portion of the lophophore is a simple arc or 

 crescent. This may be called the taxolophus. The tenta- 

 cles are few in number, and increase takes place on each 

 side of the median line, dorsally, in front of the mouth. 

 In figures 121, a, e, 122, a, f, 124, a, this character is clearly 

 shown. The tentacles at the ends of the arc are the oldest, 

 and new ones are being formed in the middle portion. 

 In Thecidea, Cistella, and Magellania the tentacles of the taxo- 

 lophus are centripetal, due to the edge of the lophophore being 

 near the margin of the shell ; while in Terebratulina, Disci- 

 nisca, and Lingula they are centrifugal, due to the smaller 

 and central lophophore. 



So far as known, there is no adult living form which has 

 the taxolophian brachial structure. It may have been pres- 

 ent in adult Iphidea of the Cambrian. 



