MOLLUSCA 



153 



of much greater strength and thickness than the others may 

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



K j' Y " p. 



ii 



Fin. 135. Diagrams of transverse sections of a Lamellibraneh to show the 

 adhesion, by concrescence, of the gill-lamelh to the mantle-Haps, to the toot, 

 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 ; 6, foot ; 

 c, mantle flap ; d, axis of gill -or ctenidium ; c, adaxial lamella of outer gill- 

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

 gill-plate ; fr, reflected lamella of inner gill-plate ; (/, line of concrescence of 

 the reflected lamelke of the two inner gill-plates ; A, rectum ; i, supra-brauchial 

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



many other Laniellibranchs, 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 



Vv 



SPfcW 



FIG. 136. Transverse, section of the outer gill-plate of Drtissena poliimnrpTia 

 (after Holman Perk). / constituent gill-filaments ; /, libr-ms sub-epidermic 

 tissue; rh, cliitonmis substance of the filaments; nch, cells relate,! to tin- 

 chitonous substance; lac, lacunar tissue; piy, pigment-cells; '-, blnod- 

 corpuscles ; Jc, frontal epithelium; lfc',lfe', tw.i r,.ws of hti-r.i-tii.ntal e\A- 

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

 in ter-filamenter 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 arc certain other points in the modification of the 

 typical ctenidium which must lie 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 



ch 



ol 



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 ; i 1 , blood-vessel ; f, constituent fila- 

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

 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 y 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 lamella? formed by the 

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

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



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 

 6 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 lamella; are represented as hav- 

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

 reflected ramus or lamella is complete (/> 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 



U 



