134 DE. T. DAVIDSON ON EECENT BEACHIOPODA. 



segment to appear being tlie median or tlioracic segment, wliicli lias probably been 

 formed by a division of the caudal segment. 



The thoracic segment carries four bundles of bristles, of which two are median and 

 two lateral ; each bundle is composed of four or five immovable cylindrical and blunt 

 bristles (woodcut, fig. 10, C). 



At the succeeding stage the mantle commences to appear under the form of two 

 folds covered with cilia, which rise from the ventral and dorsal sides of the thoracic 

 segment. 



This folding of the skin (future mantle) soon becomes developed and partly covers the 

 caudal segment. The bristles, which are then situated on this fold, become larger and 

 pass beyond the caudal segment. At the same time two pigmentary spots appear on the 

 dorsal side of the cephalic segment. 



The author was not able to observe the intermediate j^hases between this stage 

 and that where the larva issues from the incubatory pouch and swims freely. At that 

 period of the larval development the mantle covers all the caudal segment, whilst 

 the cephalic segment, which is covered with vibratile cilia, assumes an umbrella-shape, 

 at the summit of which may be distinguished a portion more or less separated from the 

 rest (head), which carries four eyes (woodcut, fig. 11, y) : this head corresponds to that 

 which M. Lacaze-Duthiers has described in Thecidium. 



The digestive tube is almost entirely confined to the thoracic segment ; its anterior 

 portion, however, pentrates a little into the cephalic segment. This last is connected 

 slightly with the adjoining segment by means of an organ of cylindrical shape. 

 Kowalevsky could not detect any traces of the nervous system. 



The muscular system of the larva is represented by delicate fibres, which pass from 

 the posterior portion of the intestine to the lateral walls of the thoracic segment. 



The mantle is composed of two layers — one external, formed of pavement-epithelium, 

 the other internal, and consisting of small cylindrical cells. Between these two layers there 

 are very delicate muscular fibres. The edges of the mantle always carry, on the ventral 

 side, four bundles of bristles, which, already existing in the preceding stages, have grown 

 and increased in length. 



At this stage the larva swims freely by the help of its vibratile cilia and by slight 

 movements of the head. At the approach of danger it vigorously contracts itself, and 

 erects its bristles in all directions. The larva, after swimming for some time, fixes 

 itself, and then the second stage of its development commences. 



Second Period. 



The attachment of the larva is effected by means of a sticky substance exuded 

 by the caudal segment ; as soon as fixed, the larva begins to turn up its mantle 

 and to raise it above its head, so that in a little time the mantle envelopes the whole of 

 the cephalic segment. On account of this foldiug-over of the mantle, the external part 

 of each lobe becomes internal, and the bristles which were on the exterior surface, 

 being thus carried inside, become unnecessary, and drop off in a few days' time. It 



