690 MOLLUSCA [LAMELLIBRANCHIA. of much greater strength and thickness than the others may be placed between each pair of groups. In Anodon, as in A FIG. 135. Diagrams of transverse sections of a Lamellibraneh to show the adhesion, by concrescence, of the gill-lamelLe 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 ; b, foot ; c, mantle 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 ; fr, reflected lamella of inner gill-plate ; g, line of concrescence of the reflected lamellae of the two inner gill-plates ; h, rectum ; i, supra-branchial space of the sub-pallial 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 lamellae v FIG. 186. Transverse section of the outer gill-plate of Dreissena pnlymorpha (after Holman reck). /, constituent gill-filaments ; /, fibrous sub-epidermic tissue ; ch, chitonous substance of the filaments ; nth, cells related to the chitonous substance ; lac, lacunar tissue ; jtiy, pigment-cells ; be, blood- corpuscles ; Jf, frontal epithelium ; Iff , Ife , two rows of latero-frontal epi thelial cells with long cilia ; irf, fibrous, possibly muscular, substance of the in ter-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 ,K " - -. UOuuO L/uC/0 o<^ oi sJUu uC/ / i^--w~^< x , -. ( J ro^E^^gspWfc u , *l ch chr 7 J lac FIG. 137. Transverse sections of gill-plates of Anodon (after Peck). A. Outer gill-plate. B. Inner gill-plate. C. A portion of B more highly magnified. o.l, outer lamella; i.l, inner lamella; v, blood-vessel; /, constituent fila ments ; /oo, 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 lamellae 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 g 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 lamellae 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-pallial chamber between the foot b and the mantle c. From it depend the gill-filaments or lamellae 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 gill-plate is fused to the mantle, whilst the inner lamella of the inner gill-plate is fused to the foot. If we pass a little backwards and take
another section nearer the hinder margin of the foot, we