778 SCIENTIFIC RECORD FOR 1885. 



In such forms as have the glands moderately developed, such as the 

 common European cockle {Cardium edule), the organ consists of "a 

 groove on the lower surface of the foot, of glands at the side of the 

 groove, of a caual which extends from it to the more or less spacious 

 cavity of the byssus, and of compact masses of byssogenous glands 

 which pass the i^roducts of their secretion into the cavity." This cavity 

 as well, and also the canal and the groove, are lined with a cylindrical 

 epithelium. In the common cockle the byssus is formed by " a simple 

 hyaline filament." 



In those forms in which the glands are more developed, such as the 

 common mussel (Mytilus), Pinna, and others, ''the glands are much 

 denser, and the anterior extremity of the foot is prolonged to form the 

 linguiform muscle; the lamellae of the cavity of the byssus are more 

 numerous," and the byssus is formed of a number of filaments consti- 

 tuting a tuft. " In Anomia differentiation is carried much further, for 

 the byssus becomes charged with carbonate of lime," and compact, so 

 as to form the so-called ossicle. 



In still other forms, "the organ undergoes great retrogression," and 

 there may be "a very short groove, or none, and no glands," as in Donax 

 and Tellina: "sometimes the groove and cavity are present, but no 

 glands exist," as in Nucula, while in others "only a delicate blind canal 

 can be distinguished, formed by a vsimple layer of epithelial cells," as in 

 Psammobia. And in few forms is there a complete absence of groove, 

 cavity, or glands. Pholas and Solen are examples of the few. 



Professor Barrels has examined more than 50 species and, accord- 

 ing to his views, representatives of "every family except the Trigoniidae 

 and Tridacnidse," and, inasmuch as the byssus and its associated struct- 

 ures are so generally developed, "he is inclined to regard it as a char- 

 acteristic organ of the Lamellibrach type." 



The so-called aquiferous pores are thought by Professor Barrois to be 

 "nothing more than the orifices of degraded byssogenous glands." It 

 is his opinion that, " if water does really enter the cirulatory system," 

 it "must do so by fine intercellular canals, or by endosmosis, or some 

 other way," but not through the aquiferous pores. {Comptes Bendus, 

 Acad. Sc. Paris, v. — , pp. 188-190 ; J. B. M. S. (2), v. 5, pp. 227, 228.) 



Structure of bivalve shells. — The bivalve shells have been considered 

 by W. Miiller with reference to the attachment existing between the 

 mantle of the animal and the shell Two varieties are recognized. In 

 one the shell is only attached at certain areas to the mantle, and "the 

 organic substance of the mother-of-pearl layer is membranous"; this 

 phase is exhibited bj almost all members of the class. In the other 

 variety the shell is "continuously grown to the mantle," and "the or- 

 ganic substance of the mother-of-pearl layer forms a network;" this 

 condition, so far as known, is only exemplified in the little fresh water 

 clam-like shells known as Sphairiidte or Cycladidse. {Zool. Anzciger, 

 V. 8, pp. 70-75 ; J. B. M. S. (2), v. 5, pp. 230^) 



