84 



BRACHIOPODA. 



of which the anterior becomes umbrella-shaped, and develops cilia 

 and eye-spots ; subsequently it atrophies and gives rise to the upper 

 lip. A fold is formed on the middle segment ; this gives rise to the 

 two mantle lobes, which soon cover the body and a part of the caudal 

 segment (fig. 556, d). Four bundles of long setae, which, as in the 

 Worms, can be drawn in and protruded, make their appearance on 

 the ventral lobe of the mantle of the developing larva. Later the 

 larva becomes attached and the metamorphosis 

 begins. The fixed posterior segment becomes the 

 peduncle ; the mantle lobes bend forward and 

 produce the shell. The bundles of setae are 

 thrown off; the deposition of calcareous matter 

 in the shell begins, and the tentacular filaments 

 (which are at first arranged in a circle) of the 

 later arms make their appearance. In Thecidiwm 

 the inner layer (mesoderm and endoderm) arises 

 from masses of cells which are budded off into 

 the segmentation cavity. The subsequent meta- 

 morphosis of the larva when provided with 

 tentacles has been most accurately investigated 

 by Brooks for Lingula, the larvae of which are 

 still free-swimming when the tentacles are being 

 developed (fig. 557, a, b). 



At the present day but few Brachiopods are 

 found in the different seas, as compared with the 

 much larger number in the earlier formations; 

 certain species of these fossil Brachiopods have 

 great importance as characteristic fossils. The 

 oldest fossils also belong to the Brachiopoda 

 and certain genera which first appeared in 

 the Silurian have persisted to the present day 

 (Lingula). 



M- 



FIG. 557 6. Longitudi- 

 nal section of an older 

 larva (after Brooks). 

 Do, Dorsal; Ve, ven- 

 tral valve of the shell ; 

 Mr, thickened mantle 

 edge ; T, tentacles ; 0, 

 mouth ; Md, stomach ; 

 Ad, intestine ; M, pos- 

 terior muscle ; G, gang- 

 lion. 



Order 1. ECARDINES (INARTICULATA.). 



Shell without hinge and brachial skeleton. Alimentary canal 

 with laterally-placed anus. Edges of the mantle lobes completely 

 separated. 



Fam. Lingulidae. Lingula anatina Lam., Indian Ocean, 

 Fam. Discinidae. Discina lamellosa Brod., South America. 

 Fam. Craniadae. Crania anomala Miill., North Sea; Cr. rostrata Hoer., 

 Mediterranean ; Cr. antiqiia Defr., fossil from the Chalk. 



