126 BULLETIN OF THE BUREAU OF FISHERIES. 



epithelium lining the cavity and also with a mechanism in its walls which allows of 

 distension, often to an extraordinary degree. 



The various types of marsupium are to be referred to differences in the manner in 

 which these compartments are associated to constitute the marsupium; to different 

 degrees to which the compartments are developed ; to differences in the modification of 

 the walls for the purpose of distension; and also to the development of special adapta- 

 tions in certain forms for increased aeration of the marsupium. Whether in the last 

 specialization the better aeration is needed for the gravid mussel, whose respiration 

 must be considerably interfered with when the entire outer gills are gorged with embryos, 

 as in Anodonta and Symphynota, or for the embryos themselves, is a question that is 

 discussed later, but from a comparison of the conditions existing in the different types 

 of marsupium it would seem that the respiratory modifications are primarily for the 

 adult and not for the embryos. The reasons for this conclusion should be reser\'ed 

 until the internal structure of the marsupium has been described. 



It is chiefly to Peck (1877) that we owe a correct interpretation of the structure 

 of the lamellibranch gill. It was he who first showed that the plate-like gills of the 

 higher forms, consisting each of an outer and an inner lamella, are formed by a series 

 of juxtaposed independent filaments, a fact that was essential to the later recognition 

 of a perfectly regular series of gradations throughout the lamellibranchs from the simple 

 ctenidium of the primitive Nucula to the complex double gill of the Unionidse. In the 

 least modified forms the filaments are straight, either plate-like or filamentous, but in 

 forms above these each filament becomes greatly elongated and bent upon itself to form 

 a compressed U or V, consisting of an inner and an outer limb. One Hmb, the inner in- 

 the outer gill and the outer in the inner gill, is fixed above to the body wall, while the 

 other limb is free in the lower groups (Area, Mytilus), fixed in the higher (Unionidse), 

 although the inner limbs, forming the inner lamella of the inner gill, may not all be 

 fused to the body wall. The filaments constituting a lamella are interlocked either by 

 cilia or by interfilamentar junctions, and the gill may be further strengthened by inter- 

 lamellar junctions, which are either simple bars (Mytilus, Margaritana) or continuous 

 septa (Unionidae, except Margaritana). 



In his study of the lamelHbranch gill Peck described in much detail and with great 

 accuracy the structure of the gills of the Unionidae, and his account has furnished the 

 basis of all subsequent descriptions. The typical structure of the unionid gill is well 

 known. Each gill consists of two lamellae, an outer and an inner, composed of series 

 of juxtaposed filaments supported by chitinous rods and fused by the interfilamentar 

 junctions except where the inhalent ostia open into the interiamellar space for the 

 entrance of water. The dorsal edge of the inner lamella of the outer gill and of the 

 outer lamella of the inner gill is fixed to the body wall, while the outer lamella of the 

 outer gill is fused to the mantle (in Margaritana it is free posteriorly), and the inner 

 lamella of the inner gill is either free or more or less attached to the visceral mass (cf. 

 Ortmann, 191 1). The two lamellae are continuous along the free ventral borders, and 

 thus form a flattened sac whose cavity opens above throughout its entire length into 



