LAMELLIBRANCHIA TA . 289 



PLATE VII. 



LAMELLIBRANCHIATA. 



FIG. i. The common Oyster (Ostrea edulis}, dissected so as to show the principal 

 features of its anatomy. 



THE animal has been removed from its shetl and dissected on its left 

 side, the one that corresponds to the flat or free valve of the shell. It may 

 therefore be compared without difficulty with the figure of Anodonta, Plate 

 vi., which has been dissected in the same way. 



a. a. a. Right lobe of the mantle which has been left entire, the left lobe 



being cut away save at its oral end. 



a'. Anterior dorsal angle. 



a". Anterior ventral angle, which is produced a little beyond the dorsal 

 angle. The part included between these two angles is fitted into a 

 deep recess in the right valve, and the ligament of the shell, which is 

 internal, corresponds to the straight edge uniting them. 



b. The two oral tentacles of the left side. The tentacles are not quite 



symmetrical in this animal. Note the deep bay which lies between 

 them and the cut edge of the left mantle which is remarkably thickened 

 in this region. % 



c. The gills or branchiae. There are four of these as in Anodonta. They 



are not symmetrical inter se, and are fluted, i. e. the lamellae are not 

 flat but undulated. 



c '. The spot where the attachment of the gills to the mantle lobes ends. 

 This attachment divides the mantle cavity into an inhalent, oral or 

 infra-branchial chamber, of great length but shallow in the Oyster, 

 and an exhalent, aboral or supra-branchial chamber, which is deep 

 and of less extent than the oral chamber. Note the four lines of 

 apertures into the interlamellar spaces of the gills. 



d. The single adductor muscle, the sole adductor present in the families 



Ostraeidae and Amculidae. The Lamellibranchiata have been divided 

 into the Monomyaria with one, and Dimyaria with two adductor 

 muscles, as in Anodonta, but the division is not a good one. This 

 single adductor corresponds to the posterior adductor of Anodonta. It 

 is distinctly divisible into two parts, an opaque portion close to the peri- 

 cardium, and a translucid portion behind. This division is observable 

 in the adductors of many Lamellibranchiata. both anterior and pos- 

 terior. Coutance has stated that the muscular fibres of the translucid 

 portion in Pecten are striated, of the opaque, smooth ; that the former 

 contract rapidly, the latter slowly; that the opaque portion is more 



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