72 



BRACHIOPODA. 



The later transformations in the lophophore have been investigated 

 in Terehratulina by ]\Iorsb. The lophojDhore here is originally 

 circular, but, later, the anterior edge becomes indented. In this 

 indentation, close to the median line, new tentacles form. The 

 tentacular apparatus, which in this way has become horseshoe- 

 shaped (Fig. 35), resembles the similar- organ in Plioronis and the 

 Phylactolaemata, this resemblance being heightened by the fact that 

 here also a dorsal fold, the lip (e), can be seen over the mouth ; 

 this must be regarded as the epistome which, in later stages, is 

 continued along the whole length of the row of tentacles. The 

 mouth (/h) lies in a ciliated furrow, the buccal groove, between 

 the ventral row of tentacles and this fold, this furrow being con- 

 tinued on to the arms as the brachial groove. The two processes 





Fig. 35. — Two ontogenetic stage.s of tlie lophophore of Terehratu- 

 lina scjitentrionalis (after Morse), a, outer, b, inner arm ; d, 

 alimentary canal ; e, epistome ; I, hepatic appendage ; in, mouth. 



of the horseshoe -shaped lophophore develop into the large lateral 

 arms of the adult, while the little, spirally-coiled, inner arms (h) 

 grow out only later in the dorsal indentation. The details of the 

 development of the lophophore in later stages must vary on account 

 of the varying shape of the adult organ in the different forms ; 

 but on this point we are still without accurate information. 



In Argiopc, the lophophore retains the simple original horseshoe -shape 

 throughout life. Both tliis form and Tliccidium show a primitive condition, 

 inasmuch as the connection of the arms with the dorsal mantle-fold is per- 

 manently retained. 



With regard to the external alteration of form in the attached 

 larva, it should further be mentioned that, according to jNIorse, tlie 



