62 EDWARD L. RICE. 



are represented solely by tufts of interlocking cilia, the ciliated 



disks (Fig. 7, B, a). The interlamellar connections are also very 



simple, consisting of hollow, cylindrical bridges, several of which 



extend between the two limbs of each adult filament (Fig. 7, 



B,b). 



Resume of Previous Investigations. 



The classic on the development of the gill in the Lamelli- 

 branchiata is the paper by Lucaze-Duthiers ('56) entitled "Me- 

 moire sur le developpement des branchies des mollusques aceph- 

 ales lamellibranches." A preliminary report ('54) preceded 

 it. The first anlage of the ctenidium is described as a row of 

 papillae arising inside the mantle, along the line of juncture of 

 mantle and body. These lengthen, bend back upon themselves, 

 and become the filaments of the inner gill. Later a second series 

 of similar papillae form just outside the first, and develop simi- 

 larly into the outer gill. Each gill thus passes through a stage 

 in which it is composed of a row of unbent filaments hanging 

 down in the mantle cavity. 



The work of Lucaze-Duthiers was preceded by two interesting 

 papers by Loven ('49), in which young bivalves of several species 

 are described and clearly figured with the ctenidium consisting of 

 from three to ten free filaments belonging to the inner gill. The 

 material was obtained from the plankton ; and the identification 

 [Mya, Tellina, Mytilus, Mactra, and Nucula are hesitatingly named) 

 was recognized as very questionable by Loven himself. More- 

 over, little account of the development is given. 



Since the time of Loven and Lacaze-Duthiers the investigation 

 of gill development has been extended to a considerable number 

 of widely separated genera of lamellibranchs. 



Early stages of Cyclas were studied by O. Schmidt ('54); and 

 his work on this genus has been extended by Leydig ('55), 

 Stepanoff ('65), and Ziegler ('85), while Lankester ('75) gives 

 brief notes on the closely allied genus Pisidium. 



Unio and Anodonta have been described by Braun ('78), 

 Schierholz ('y8, '89), and F. Schmidt ('85). 



The earliest stages of the gill of Teredo have been described 

 briefly, and not very clearly, by Hatschek ('80). Sigerfoos ('96) 

 has confirmed the observations of Hatschek and extended them 

 to much later stages. 



