MOLLUSCA 



153 



of much greater strength and thickness than the others may 

 be placed between each pair of groups. In Anodon, as in 



Fig. 135. — Diagrams of transverse sections of a Lamelliljranch to sliow the 

 adhesion, by concrescence, of the gill-lamellae to the mantle-flaps, to the foot, 

 and to one another. A shows two conditions with free gill-axis ; B, con- 

 dition at foremost region in Anodon ; C, hind region of foot in Anodon ; D, 

 region altogether posterior to the foot in Anodon, a, visceral mass ; &, foot ; 

 c, mantlC' flap ; d, axis of gill or ctenidium ; e, adaxial lamella of outer gill- 

 plate ; er, reflected lamella of outer gill-plate ; f, adaxial lamella of inner 

 gill-plate ; Jr^ reflected lamella of inner gill-plate ; g, line of concrescence of 

 the reflected lamellse of the two inner gill-plates ; h, rectum ; i, supra-branchial 

 space of the sub-paUial chamber. (Original.) 



many other Lamellibranchs, the ova and hatched embryos 

 are carried for a time in the ctenidia or gill apparatus, and 

 in this particular case the space between the two lamellse 



/ / / 1- J^^ v^^' 



f f f f / 



Fio. 136.— Transverse section of the outer gill-plate of Drdssma polymorpha 

 (after Holman Peck), f, constituent gill-fllaments ; /^ fibrous sub^epidermio 



thelial cells -with long cilia ; Irf, fibrous, possibly muscular, substance of the 

 inter-fllamenter junctions. 



of the outer gill-plate is that which serves to receive the 

 ova (fig. 137, A). The young are nourished by a substance 



formed by the cells which cover the spongy inter-lamellar 

 outgrowths. 



There are certain other points in the modification of the 

 typical ctenidium which must be noted in order to under- 

 stand the ctenidium of Anodon. The axis of each ctenid- 

 ium, right and left, starts from a point well forward near 

 the labial tentacles, but it is at first only a ridge, and does 

 not project as a free cylindrical axis until the back part of 



A 



Fig. 137.— Transverse sections of gill-plates of Anodon (after Peck). A. Outer 

 gill-plate. B. Inner giU-plate. C. A portion of B more highly magnified. 

 o.lt outer lamella ; i.l, inner lamella ; v, blood-vessel ; f, constituent fila- 

 ments ; lac, lacunar tissue ; ch, chitonous substance of the filament ; chr, 

 chitonous rod embedded in the softer substance ch. 



the foot is reached. This is difficult to see at all in Ano- 

 don, but if the mantle-skirt be entirely cleared away, and 

 if the dependent lamellse which spring from the ctenidial 

 axis be carefully cropped away so as to leave the axis itself 

 intact, we obtain the form shown in fig. 131, where ff and 

 h are respectively the left and the right ctenidial axes pro- 

 jecting freely beyond the body. In Area this can be seen 

 with far less trouble, for the filaments are more easily re- 

 moved than are the consolidated lamellse formed by the 

 filaments of Anodon, and in Area the free axes of the 

 ctenidia are large and firm in texture (fig. 132, c, d). 



If we were to make a vertical section across the long 

 axis of a Lamellibranch which had the axis of its ctenidium 

 free from its origin onwards, we should find such relations 

 as are shown in the diagram fig. 135, A. The gill axis d 

 is seen lying in the sub-paUial chamber between the foot 

 b and the mantle c. From it depend the gill-filaments or 

 lamellse — formed by united filaments — drawn as black lines 

 /. On the left side these lamellae are represented as hav- 

 ing only a small reflected growth, on the right side the 

 reflected ramus or lamella is complete (fr and er). The 

 actual condition in Anodon at the region where the gills 

 commence anteriorly is shown in fig. 135, B. The axis of 

 the ctenidium is seen to be adherent to, or fused by con- 

 crescence with, the body-wall, and moreover on each side 

 the outer lamella of the outer giU-plate is fused to the 

 mantle, whilst the inner lamella of the inner giU-plate is 

 fused to the foot. If we pass a little backwards and take 

 another section nearer the hinder margin of the foot, we 



u 



