68 BRACHIOPODA. 



of the larva. The latter region gives rise to the peduncle of the 

 adnlt. 



The eggs of Thecidium, which are distinguished by their com- 

 paratively large size, after leaving the oviduct, pass into a brood-sac 

 which develops as a median invagination of the ventral mantle-lobe, 

 into which two of the cirri of the ring of tentacles hang down. To 

 these the eggs are attached by means of fine filaments (cf. Pliownis). 

 Cleavage here also is total and equal, but the cleavage-cavity is from 

 the first small. An invagination of the blastoderm does not here 

 take place, but the second embryonic layer arises " through the 

 simple and irregular formation of its cells from the cells of the 

 blastoderm," and thus proliably by polar ingression. The whole 

 of the cleavage-cavity is soon filled with cells of the primary 

 entoderm, which become arranged into three masses, in each of 

 Avhich a cavity soon appears. The part that lies between the other 

 two becomes the mid-gut, and the lateral parts represent the 

 coelomic sacs, so that we now have a stage equivalent to that 

 described above for Argiope. The further development of tlie two 

 forms also agrees. The embryo first lengthens and becomes divided 

 up into transverse regions. According to Lacaze-Duthiers, the 

 middle region here arises by abstriction from the anterior half. 

 The most anterior part of the cephalic region becomes marked off 

 later by a circular furrow, so that the ciliated larva is finally com- 

 posed of four distinct regions separated from each other by circular 

 furrows. 



B. Metamorphosis. 



Our knowledge of the metamorphosis of the Brachiopoda we owe- 

 principally to the investigations of Morse in Terehrafulina, and 

 KoWALEVSKY in Avgiope and Thecidmm, but these investigations 

 are far from complete. The attachment of the larva, the form of 

 which has been briefly described above, is brought about by means 

 of a cement secreted by tlie posterior pole of the body (pedal 

 region). The mantle -fold now bends anteriorly (Figs. 31 F-K, 

 32 A), so that it soon completely envelops the cephalic segment. 

 The former external surface of the mantle-lobes now becomes the 

 inner surface and vice versa. In this way the points of insertion 

 of the four larval tufts of setae come to lie on the inner surface of 

 the mantle (Fig. 32). The setae now soon fall off (Fig. 33), and, 

 in those forms which, in the adult condition, possess setae on the 

 margin of tlie mantle, are replaced by the permanent setae. In 



