68 



LAMELLIBRANCHIA. 



E. The Gills. 



In those Lamellibranchs in which the formation of the gills has 

 heen studied, they are found to arise in one of two [three, of. p. 45 j 

 different ways which are somewhat difficult to harmonise in then- 

 early stages. According to one method, which has already been 

 described for Gyclas and Teredo (pp. 42 and 44), a fold resembling the 

 mantle-fold rises between the latter and the foot, and develops from 

 behind forward. The outer and inner surfaces of these folds show 

 oroove-like depressions lying at right angles to the longitudinal axis 

 of the folds ; these grooves deepen and, meeting those of the opposite 

 surface fuse together. As the gill-fold becomes perforated along 

 these lines, fissures result which extend in from the free margin 

 of the folds towards their bases (Fig. 31, p. 75). The gill now con- 

 sists of a series of consecutive lobes which decrease in size from 



ltefore backward. . 



According to the other method of gill-formation, which has been 

 observed in Myttius, Dreissensia, Ostrea (a somewhat similar method 

 being found also in the Uvionidae),* a papilla arises on each side of 

 the body between the mantle and the median visceral mass, and 

 behind these new papillae arise (Fig. 26 C). A longitudinally 

 placed row of papillae thus arises by the continued development of 

 fresh papillae behind those already formed. These, by the develop- 

 ment of interfilamentar junctions, form the inner branchial leaf 

 whi le the outer leaf is produced by a similar row of papillae which 



arise somewhat later. 



The further development of the papillae was studied by L-acaze- 

 Duthiers in a form belonging to the last category, viz., m Mytilus 

 edulis (No 28). Jackson also has recently investigated the forma- 

 tion of the gills in Ostrea, and has arrived on the whole at the same 

 results as Lacaze-Duthiers (No. 22). 



During the development of the inner branchial leaf, the papillae 

 increase in number, new ones continually budding out posteriorly. 



* This seems also to be indicated by the observations made by Loven on 



mmmmmm 



the primitive condition. 



