54 THURLOW C. NELSON 



scientists than this singular organ, and j^et, where described in 

 many of the modern text-books, its function is ignored or held 

 more or less doubtful. It seems that we really know little more 

 regarding the biological role of the crystalline style than we did 

 two centuries ago. A survey of the literature show ^ that the con- 

 clusions of the many investigators are decidedly contradictory. 

 Much of the evidence advanced is not well supported and the 

 conclusions in many instances are mere guesses. 



It is to be noted, furthermore, that some recent authors (von 

 Fiirth, '03; Gutheil, '11; Matthias, '14; Allen, '14, and others) 

 have accepted the work of Mitra ('01) as conclusive. The inves- 

 tigations of this scientist were comprehensive and carefully con- 

 ceived as far as they were carried out. However, as we shall see 

 later, much of the credit given to Mitra rightfully belongs to 

 earlier workers. He gives no survey of the literature beyond the 

 theories presented in four text-books, and some of his conclusions 

 are untenable in the light of more recent investigations. 



The present contribution is an attempt to bring all known 

 data under one head and to attack the problem from everj- pos- 

 sible angle. 



My own investigations, together with such results of other 

 workers as are applicable, will be considered under the main 

 topics of morphology, histology, physiology, embryology, ecol- 

 ogy, and evolution. 



The crystalline style 



In figure 1 is shown a fresh-water mussel, Anodonta grandis, 

 dissected to show the crystalline style and the part of the ali- 

 mentary canal in which it is lodged. As these structures as found 

 in this species are fairly typical of the more common lamelli- 

 branchs, Anodonta will be used as the type form. 



The intestine leaves the posterior end of the stomach and runs 

 in a posteroventral direction along the base of the foot to w^ithin 

 a short distance of the posterior margin. Here it turns dorsad 

 and, making a wide bend, continues dorso-anteriorly through the 

 hepato-pancreas to a point below the anterior end of the heart. 

 Here the intestine makes a backward turn upon itself, and after 



