172 BULLETIN OF THE BUREAU OF FISHERIES. 



The relations of the mantle as observed will have greater significance from a state- 

 ment of its functions. Besides supplementing the gills in respiration and serving along 

 its border as a sensory organ, a chief function of the mantle is the formation of shell. 

 The extreme edge of the mantle secretes the homy covering of the shell, as also the liga- 

 ments and lunule, while the remaining mantle surface secretes the calcareous shell. 

 For our purpose, accordingly, the mantle is a most significant organ. Diseases or other 

 influences affecting the mantle frequently show effects in the shape, color, or quality 

 of the shell, and it is in the mantle, probably, that all free pearls are produced. The 

 mantle is not, however, the only portion of the mussel capable of forming shell. The 

 two adductor muscles pass entirely through the mantle, having direct attachment to the 

 shell. While the shell becomes thicker in other parts by the superposition of layer after 

 layer of calcareous material from the surface of the mantle, the thickening of the shell 

 against the muscles is in some measure, apparently, a function of the muscles them- 

 selves. It is not surprising, therefore, that these muscles also give rise to a large number 

 of pearl formations, baroques, and slugs, but not, ordinarily, good pearls. No other 

 parts commonly give origin to pearls, although it is reported that pearls have been 

 found within the body. Baroque pearls and slugs are frequently found in the tissue 

 just beneath the hinge line, but this is actually a part of the mantle. 



The shell substance formed by the muscles is called hypostracum, and is largely 

 horny in nature. Since each muscle occupies a nearly constant relative position regard- 

 less of the size to which the mussel attains, it is evident that in any adult individual the 

 muscle traveled in the course of life history from the back to its latest position; the 

 hypostracum, therefore, does not occupy a single spot but is a tapering vein passing 

 through the nacre from the beak to the position of the muscle at any given time. Simi- 

 larly the hypostracum of the pallial line is the margin of a thin stratum of like sub- 

 stance which extends from the beak or beginning of the shell and divides the nacre 

 into two portions (p. 130). 



The mantle has other functions of great importance. When the muscles are relaxed 

 and the shell is gaping, the opening between the valves of the shell is largely closed by 

 the apposed margins of the mantle. Nothing can enter between the valves of the shell 

 without affecting the highly sensitive border of the mantle and thus giving warning 

 to the animal, which may then contract its muscles and close the shell instantly. The 

 nerves of the margin of the mantle are not only sensitive to tactile stimuli, but apparently 

 are also connected with organs of something like visual function, so that the animal 

 may close or open its shell under the influence of shadows or bright light. 



It is the margins of the mantle that surround and form the two siphonal openings 

 at the hinder end of the shell, through one of which water and food pass into the shell, 

 while through the other water passes out, conveying the waste products. The lower 

 of these two openings particularly is protected by projections of the mantle, in the form 

 of papilla; or fimbriae, which, being very sensitive, give warning of any objectionable 

 character or content of the water. 



OTHER CONSPICUOUS ORGANS. 



Without disturbing the upper mantle two internal organs are distinctly evident. 

 The heart is recognized by its throbbing action. It lies at the back just below the lateral 

 teeth of the hinge and in front of the posterior adductor muscle. The rate of beating 



