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BULLETIN OF THE 



Diagrammatic Sketch of different Forms of the Ctenidia in Mollusks. 



The length of the filaments or lamellae in the first five figures of the diagram is made 

 for convenience disproportionally small. 



A. Cross-section of gill of Dimya, showing large blood-vessel in the stem, and the 

 position occupied by the filaments upon the stem. 



B. Cross-section of the gill in Amusium Dalli, the filaments touching but not organically 

 united above. 



C. The same of Area ectocomata, showing the tubular filaments planted in a groove, 

 and the asymmetrical position of the blood-vessels. 



D. Section of gill in Area Noce, the filaments organically united. 



E. The same for Janira hemicyclica. 



F. Cross-section of one of the gills of Pleurotomaria Adansoniana. 



G. Gill in Cardium sp. 



H. Cross-section of the left pair of gills in Perna ephippium ; at a these are united with 

 the right-hand pair by connective tissue crossing the median line of the animal. 

 I. Cross-section of the gill in Nucula, after Mitsukuri. 



Dimya we have the still simpler form of a stem with only a single series of 

 filaments. (See Diagram A.) In Amusium Dalli we have a gill of the form 

 described by Prof. Huxley. The stem has two series of filaments, which are 

 organically connected only at their bases, one series attached on each side of 

 the stem with the space between them slightly excavated, and the lumen of the 

 blood-vessel below it of a semilunar shape. (See Diagram B.) The filaments, as 

 in Dimya, are adhesive to one another wherever they touch, have hoof-shaped 

 extremities, and are supported by fine chitinous rods, one to each filament. 

 In the ordinary shallow- water Pectens (e. g. Janira hemicyclica, see Diagram E) 

 there are on each side of the stem two series of filaments, organically con- 

 nected by delicate tissue, sustained by rods of chitine, and forming a double 

 series on each side of the stem. These series do not adhere to one another, 

 but the tips of the filaments in Amusium Dalli do adhere over the groove by 

 the adhesiveness of their ciliated surfaces, and practically form a single sac, 

 comparable to one of the two series observable in Janira ; but, composed of the 

 elements of which the inner halves of the series in Janira are made up, while 

 the respective outer halves are absent in the Amusium. 



It may of course be claimed that the two series on each side of the gill in 

 D and E are merely the single series of C and B laterally extended, so as to 

 form a flat lamella, instead of a columnar filament, for each process. Indeed, 



