74 



BRACHIOFODA. 



O— J-- 



The youngest larva of Lingula observed by Brooks vpas already 

 enclosed in two flat, discoidal shell-valves, not articulating with one 



another, but covering the animal dorsally and 

 ventrally, their edges being free all round (Fig. 

 37, s and t). The anterior part of the mantle- 

 cavity is occupied liy the disc-like lophophore 

 beset with tentacles (Figs. 36 and 37), in the 

 centre of which can be recognised the oral 

 aperture (Fig. 36, in) and an epistomal fold 

 (e) overhanging it. In the lophophore, a 

 striking feature is the presence of a dorsal, 

 unpaired tentacle {t), at the sides of which the 

 youngest tentacle -rudiments grow out (Fig. 

 36, t'). The actual body of the animal is 

 small, and contains a body-cavity Avhich is not 

 spacious, an alimentary canal, and a few groups 

 of muscles. Among these latter, we notice first 

 a pair running near the oesophagus (oe) and an 

 unpaired liundle extending from shell to shell. 

 The alimentary canal at first appears divided 

 into the bent oesophagus (oe), a dilated stomach 

 (r?), in connection with which can soon be seen 

 the hepatic diverticula and a posterior intestinal 

 outgrowth ((?'), which bends round anteriorly 

 and fuses with the body-wall on the right side, 

 at which point the future anal aperture soon 

 arises (Fig. 36, a). 



Other features to be noted in the youngest 

 Lingula larva are the complete absence of the 

 rudiment of the j^eduncle and the presence of 

 a remarkable semicircular skeletal plate, which 

 lies below the dorsal shell-valve and is con- 

 nected with that valve. In the larva described 

 by F. MtJLLER (Fig. 38), which perhaps belongs 

 to Crania, five pairs of strong provisional setae 

 could be seen, the larva creeping by the help 

 lateral movements of the shell -valves. In 

 the shell -valves are opened and the lophophore is 

 extended far beyond the shell, the swimming movements being 

 brought about by the cilia covering the tentacles. 



The rudiment of the nervous system can be recognised in the form 



Fig. 37. — Diagrammatic 

 liiedian section through 

 the Liwjula larva (after 

 Brooks, from Bal- 

 four's Text -book), a, 

 margins of the shell- 

 valves ; h, thickened 

 margin of the mantle ; 

 (', mantle ; d, median 

 dorsal tentacle ; c , 

 lophophore ; /, epis- 

 tome ; g, mouth ; h, 

 mantle - cavity; i, 

 body - cavity ; k, wall 

 of the oesophagus ; I, 

 oesophagus ; m, hepatic 

 chamber of the stom- 

 ach; H, intestinal cham- 

 ber of the stomach ; o, 

 hind -gut; q, ventral 

 ganglion ; r, posterior 

 muscle ; s, dorsal, t, 

 ventral shell-valve. 



of these and the 



swmimnig. 



