SEA MUSSEL MYTILUS EDULIS. 



167 



particles and sweep them toward the food groove on the ventral edge (fig. 152). On 

 the interlamellar or ab-frontal surface the cilia sweep upward or in just the opposite 

 direction of the frontal cilia. They serve to help in producing the main current and 

 in keeping the inner surfaces of the gills clean. 



The long latero-frontal cilia are undoubtedly the straining mechanism. They pro- 

 ject out from the sides of the filaments, forming a sieve, and lash relatively slowly 

 across the middle of the frontal face of the filament (fig. 152; fig. 149, p. 165). Orton 

 (1912) summarizes his results as follows: 



Thus Nucula and Mytilus have four kinds of cilia, the lateral cilia producing the main current, 

 the frontal for collecting and transporting the food, the fronto-lateral which assists in food collecting 

 and the ab-frontal or inner cilia which help in producing the main current, in collecting food, and in 

 cleaning the filaments. 



The gland cells which Kellogg (1892) says are present in the latero-frontal region 

 of the filaments probably serve to secrete a mucus which cements the food particles 



FC LFC LC 



152 



F1G.152. — Longitudinal interfilamentary view of a living filament of the left outer lamella of the gill. X 84 approximately 

 a, b, c, arrows indicating roughly the directions in which the latero-frontal. lateral, and frontal cilia, respectively, lash; CJ, ciliary 

 junction; FC, frontal cilia; FG, food groove; LC, lateral cilia; LFC, latero-frontal cilia. 



together in morsels of convenient size. These are swept by the frontal cilia into the 

 food groove, in which they are carried forward by the cilia lining its walls to the labial 

 palps, which transfer them into the mouth. 



As an organ of respiration the gills perform their function incompletely. This was 

 recognized by Sabatier (1874), who found that circulation took place within the fila- 

 ments in a very imperfect manner. The defective circulation, according to this 

 author, is due (1) to the small caliber of the branchial vessels, (2) to the weak cur- 

 rent of the blood which flows to the gills after having traversed the kidney or other 

 capillary network, and (3) to the existence of other and larger channels which allow the 

 blood to return to the heart without traversing the gills. 



The mantle serves as an organ of respiration when it is not distended with genital 

 products. During the period of reproductive quiescence it is a thin-walled organ, with 

 the blood vessels separated from the outside medium by a very thin layer of tissue. 

 During reproductive activity, however, the walls of the mantle become thick and the 

 blood vessels are covered with heavy layers of tissue in which metabolic activity is 



