48 DR J. H. ASHWORTH 



tips of the teeth which lie right and left of the hinge-line on the ventral valve is 

 *32 mm. These measurements remain unchanged throughout larval life, for sub- 

 sequent additions to the valves take place on their lateral and anterior margins only. 

 The straight hinge-line, in all the later stages examined, is about "28 to '32 mm. 

 long (average of the whole series, "295 mm.), there being some individual variation. 



The margins of the primary shells, or protegula,* are well seen in the dorsal 

 and ventral valves, but especially in the latter, and remain recognisable in all the 

 later larval stages examined. The two protegula are unequal ; the dorsal one (fig. 2) 

 is almost, but not quite, a semicircle of which the hinge-line, "3 mm. long, is the 

 diameter, its length or height being - 14 mm.; the ventral one is smaller (fig. 1), 

 its length being *12 mm. When the formation of the secondary shell takes place, 

 by additions to the lateral and anterior margins of the protegula, the two valves 

 soon become approximately equal in size. In all my larvae the two valves are either 

 equal in size or the ventral one is slightly larger, as it is in the adult. 



This larva has 8 pairs of ciliated cirri. The most anterior cirrus of each side 

 is smaller than the rest, and has been recently formed ; it lies immediately lateral 

 to the base of the median sensory tentacle. This tentacle is an outgrowth from the 

 lip-like fold (epistome) which overhangs the mouth in front. 



The mouth leads into the oesophagus and mid-gut. The mid-gut is a wide sac, 

 from the walls of which the lobes of the digestive gland or liver — a right and left 

 posterior dorsal and a bifid ventral lobe f — are being formed. The intestine is only 

 feebly differentiated, and the annus is not yet present. The anterior occlusor muscles 

 are of moderate size, but the posterior occlusor has not yet appeared. 



Two statocysts i are present in the dorsal body-wall, each situated a little 

 posterior and median to the occlusor muscle of its side (see below, p. 51). 



The organs practically fill the ccelom, there being usually only small portions 

 of the cavity recognisable right and left of the oesophagus and in the middle line 

 at the posterior end. 



There is no trace of peduncle. In regard to. its internal organs, this larva has 

 reached a stage of development similar to the larva with 5 pairs of cirri, represented 

 in Dr Yatsu's figs. 77, 78. 



for the valves. In the specimen -52 mm. long, described above, the dorsal and ventral valves are still connected 

 together posteriorly by the fold. In all the later stages examined, the fold, while remaining attached to the ventral 

 valve, has become disconnected from the dorsal valve ; it still, however, bends over dorsally so that its edge often 

 engages with the now thickened, straight, posterior margin of the dorsal valve. This margin, which corresponds 

 with that of the ventral valve, is the hinge-line referred to above. 



* See footnote on previous page. 



t For details of these lobes, see p. 52. 



% Statocysts were first seen in Brachiopods by Fritz Muller (1860, p. 77), who observed them in larv;e of 

 PelarjorHscus. Professor MORSE (1878, p. 266) recorded the presence of statocysts in adult Lingula, and Brooks 

 (1879, p. 63) saw them in larvw of Glottidia. Professor Blochmann (1898, p. 422 ; 1900, p. 124), however, denied the 

 presence of statocysts in both Lingula and Pdagodiscw, and considered the structures in question to be the funnels 

 of the nephridia. Dr Yatsu (1 902, pp. 64-68) held that they were statocysts, and, after having examined these organs 

 in 1 iving and preserved specimens of both Lingula and Pelagodiscus, I can fully confirm his view. The funnels of the 

 nephridia have a totally different appearance and different relations (see figs. 8, 10). 



