164 



BULLETIN OF THE BUREAU OF FISHERIES. 



Our first knowledge of the finer structure of the gills of Mytilus is due to the careful 

 work of Lacaze-Duthiers (1856), who studied both their constitution and development. 

 The nomenclature which he introduced will be used in the description which follows. 

 When observed in cross section the gills appear in the form of a narrow W suspended 

 by the upper angle with the outer lamellfe terminating with a free edge in the mantle 

 cavity (fig. 147, Br, opp. p. 163). This gives a branchial apparatus on each side of the 



body composed of two folds, each of which is made up of 

 two lamellaj. The outer plates on each side are known as 

 '"•TinV ^ e "S^t or ' e ^ t outer gill plates, and in like manner the 

 inner plates are designated as the right or left inner gill 

 plates (fig. 148). The separate lamella? of each plate are 

 known as the descending portions, which arise from the 

 point of attachment of the gills and pass to the ventral 

 edge, and the ascending portions, which arise from the ven- 

 tral border and extend upward terminating with a free 

 edge. The space inclosed between the ascending and 

 descending lamella? is known as the interlamellar space 

 (fig. 148, IS). 



In side view the gills are seen to be composed of 

 numerous parallel filaments which lie one next to the other, 

 with a slight interfilamentar space between them. They 

 are cross-marked with fine, light-colored striations which 

 form parallel lines running from the anterior to the pos- 

 terior end of the gills. 



HISTOLOGY. 



The filaments which make up the gills are composed of 

 specialized groups of ciliated epithelial cells which surround 

 a central canal or branchial blood vessel (fig. 149). In 

 this cross section of a gill filament four types of cilia may 

 be clearly distinguished: (1) Frontal cilia, which are rela- 

 tively short (FC) ; (2) latero-frontal cilia, which are very 

 long (LFQ ; (3) lateral cilia, also very long (LCj ; and (4) 

 ab-frontal cilia, which are the least developed of all (AFC). 

 Each group has a special function to perform, as will be ex- 

 plained later. Gland cells are also present in the latero- 

 frontal region, according to Kellogg (1892). The author's 

 own preparations do not show them unless the ciliated cells 



which bear the latero-frontal and lateral groups of cilia also function as gland cells. 



Their protoplasmic content is filled with fine granules which stain deeply with acid 



fuchsin when no other parts of the tissue take this dye. They have the appearance 



of gland cells, but at the same time seem to be nothing more than highly specialized 



ciliated cells. 



Connections between the filaments are established by means of tufts of cilia which 



project from their anterior and posterior surfaces close to their interlamellar edge. 



These ciliated junctions occur at short intervals over the entire length of both the 



148 



Transverse section of a gill. ABV, 

 afferent branchial vein; AP, ascending 

 portion of gill lamella; CJ, ciliary junc- 

 tion; DP, descending portion of gill 

 lamella; EBV , efferent branchial vein; 

 FG, food groove; //, interlamellar junc- 

 tion; IS, interlamellar space; PIC, pli- 

 cate canal; RnV, renal veins lined with 

 kidney tissue. 



