158 



BULLETIN OF THE BUREAU OF FISHERIES. 



At a very early stage a special organ of attachment is formed in some species, espe- 

 cially among the Lampsiliniae (Sterki, 1891, 1891a; Frierson, 1903, 1905; and Lefevre and 

 Curtis, 1912). This is the byssus, a sticky hyaline thread produced by a byssus gland 

 formed in the middle line of the rear portion of the lower side of the foot. In the wash- 

 board, Quadrula keros, a very few days after leaving the fish there is apparent a tough 

 mucuslike secretion by means of which the juvenile mussel may anchor itself. The 

 byssus may serve to anchor the mussel by attachment to foreign objects, but its func- 

 tion needs to be more definitely ascertained. Juvenile mussels are sometimes captured 

 in considerable numbers, owing to the sticky thread becoming attached or entangled on 

 the crowfoot hooks or lines or on aquatic vegetation drawn into the boat. While such 

 observations suggest the function of keeping the mussel from being carried away by 

 the current, nevertheless the organ is well developed in young Lake Pepin muckets 

 which are observed to bury themselves deeply in the bottom. The byssus is retained a 

 varying length of time in different species and in different individuals of the same species. 

 ' The byssus has been seen in young muckets, Lampsilis ligamentina, late in the second 

 year of free life and rarely in adults of Plagiola donaciformis. The species of mussel 

 observed with byssus are listed below. 



SPECIES OF MUSSELS THE JUVENILES OF WHICH ARE KNOWN TO HAVE A BYSSUS. 



Lampsilis alata, pink heel-splitter. 



L. anodontoides, yellow sand-shell. 



L. capax, pocketbook. 



L. ellipsiformis. 



L. fallaciosa, slough sand-shell. 



L. gracilis, paper-shell. 



L. iris, rainbow- shell. 



L. laevissima, paper-shell. 



L. ligamentina, mucket. 



L. luteola, Lake Pepin mucket. 

 L. recta, black sand-shell. 

 L. ventricosa, pocketbook. 

 Obovaria ellipsis, hickory-nut. 

 Plagiola donaciformis. 

 P. elegans, deer-toe. 

 Quadrula ebenus, niggerhead. 

 Q. plicata, blue-point. 



The shell formed during the first month (more or less) of development possesses 

 certain peculiar characteristics — besides having a relatively low lime content and being 

 transparent, it bears on its surface certain relatively high ridges, knobs, etc. (PI. XX). 

 The cause or the meaning of these nicely formed ridges is unknown, but the pattern of 

 sculpture of the early juvenile shell is characteristic for the species. Though all the 

 remainder of the shell be perfectly smooth, the "umbonal sculpture," as it is called, can 

 be made out in well preserved adult shells of most species, and their markings are given 

 significance in the classification of mussels. 



We need not concern ourselves here with the details of development of the internal 

 organs, except to say that a considerable elaboration of structure must ensue before 

 the mussel is prepared to assume its culminating function — the reproduction of its 

 kind. The first act of breeding marks the close of the juvenile period, and this occurs 

 in the Lake Pepin mucket two years after the beginning of the juvenile stage, or early 

 in the third summer of life counting from the deposition of the egg in the gill of the 

 mother. In some species of mussels, those of small adult size, or those possessing very 

 thin shells, sexual maturity comes at an earlier age, but in most species of mussels it 

 undoubtedly occurs later. (See p. 137.) 



The maximum sizes, at various ages, attained by Lake Pepin muckets under obser- 

 vation, are shown in the following table: 



