scHrfnERT.] CLASSIFICATION OF BRACHIAL STRUCTURES. 107 



useful, also, in desiguatiiig the kind of brachial coniploxity attained 

 in any genus the arm stTU(;turc of which can bo determined, thus help- 

 ing,' to lix its place in a genetic scale. It should bo emphasized, how- 

 ever, that the form and complexity of the cirrated margin of thelopho- 

 phore can liave a taxonomic value only within i-omparatively narrow 

 limits. This at once becomes evident when the arms of liingula, Dis- 

 cinisca, Crania, Ehynchonella, and all the Spiriferacea are considered. 

 Each has spiral arms, which were lu-obably develo]>ed through similar 

 changes of form, and yet each is genetically distinct, as shown by all 

 the other leading characters. But when this classification of arm 

 structures is ap]>lied within a family or genus, or even when made 

 the basis of comjiarison among some closely related families, it is some- 

 times 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 tentaciles, 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 Jeioloplnis 

 stage, though it has no special significance beyond indicating the 

 beginning of the lophophore. 



TAXOLOPHXTS STAGE. 



The first stage in which a true brachial structure is manifest 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 

 taxoIopJius. The tentacles are few in number, and increase takes place 

 on each side of the median line, dorsally, in front of the mouth. In 

 figs. 2a, €j 3rt, /, 5rt. 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 Magellaiiia the tentacles of 

 the taxolophus are centripetal, due to the edge of the lophophore being- 

 near the margin of the shell; while in Terebratuliua, Discinisca, and 

 Lingula they are centrifugal, due to the smaller and central lophophore. 



So Air as known, there is no adult living form which has the taxolo- 

 phian brachial structure. It may have been present in adult Iphidea 

 of the Cambrian. 



TROCHOLOPHUS STAGE. 



By the continual addition of new cirri and the pushing back of the 

 old ones, the fringed margin of the lophophore ])asses from a crescentio 

 to a circular form, thus making a complete ring about the mouth. This 

 may be termed the trocholophus stage. It appears in the late larval and 

 early adolescent stages of Tliecidea (fig. 2?>), Cistella {^v^. 2/), IMagella- 

 uiaand Terebratalia (fig. '■\b), Terebratuliua (fig. 3^), (Jlottidia (fig. 5^), 



