68 LAMELLIBBANCHIA. 



E, The Gills. 



In those Lamellibranchs in which the formation of the gills has 

 been studied, they are found to arise in one of two [three, cf. p. 45] 

 different ways which are somewhat difficult to harmonise in their 

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

 described for Cyclas 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 

 groove-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 

 before backward. 



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

 observed in Mytilus. Dreissensia, Ostrea (a somewhat similar method 

 being found also in the Umonidae),* 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, 

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

 arise somewhat later. 



The further development of the papillae was studied by LACAZE- 

 DUTHIEBS in a form belonging to the last category, viz., in 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 

 Montacuta. The filiform permanent gill, of Pecten at any rate, arises as 

 papillae, and BAY LANKESTEB states that the gills of Pisidium appear first in 

 the form of papillae, although these, from the figures, at first look like the 

 mere swellings of a fold. These statements recall the condition in the nearly 

 related genus Cyclas, in which also papilla-like structures produced by the 

 splitting of a leaf are found as the rudiments of the gills. It is, however, 

 possible that Pisidium, in the formation of its gills, may be somewhat nearer 

 the primitive condition. 



