312 ANIMAL BIOLOGY. [Part II. 



the region of B. the supra-branchial chamber is sub-divided into 

 four parts, one for each gill-plate. 



Eegarding, then, the gills as composed of a great number ot 

 filaments with descending and ascending moieties and we are 

 justified by comparative morphology and development in so re- 

 garding them we have to note the large amount of fusion or 

 concrescence of parts. 



1. The inner surfaces of adjoining filaments have become 

 united by a membranous expansion, the lamellar membrane, 

 perforated with oval fenestrae or windows. Thus the ascending 

 and descending filaments respectively come to form ascending 

 and descending lamellae. 



2. The two lamellae of each gill become fused by inter- 

 lamellar junctions, occurring in the inner gill at intervals of 

 about twenty filaments, and in the outer gill at intervals of 

 seven or eight filaments. 



3. The ascending lamella of the outer gill is concrescent with 

 the mantle along its whole length. 



4. The ascending lamella of the inner gill is concrescent 

 anteriorly with the foot. 



5. The ascending lamellae of the inner gills are concrescent 

 with each other posteriorly. 



6. The union of the gill-axis with the body-wall may be 

 partly due to concrescence. 



Two little square patches should be cut out from the descend- 

 ing (outer) lamella of the inner gill of a recently-killed mussel, 

 and be mounted in water (without cover glass), the one with its 

 outer and the other with its inner face uppermost. Examined 

 under a moderate power of the microscope, that with its outer 

 side uppermost shows a number of parallel bars the gill- 

 filaments strengthened with yellowish chitinous rods. Where 

 there is a fenestra in the underlying lamellar membrane, the 

 play of numberless cilia, working in one direction on one fila- 

 ment, and the opposite direction on the other, may readily be 

 watched. The patch with its inner face uppermost, shows 

 (especially with reflected light) the oval fenestrae in the lamellar 

 membrane. If some of the gill substance be teased and 



