430 MARINE INVERTEBRATES 



have become united at their ends to the mantle (on the outer gills) 

 and to the body-mass (on the inner gills). Here also the first traces 

 of the higher type of gill found in the next order appear ; that is, a 

 gill in which the filaments are connected by vascular channels in- 

 stead of by cilia. This order therefore is presumed to connect 

 the true filamentous t} 7 pe of gill with the fully developed lamellar 

 type. As the filaments do not always fuse with the mantle or 

 foot, the description of the pseudolamellibranch gill becomes 

 somewhat vague, and there is some room for doubt whether this 

 order should exist at all. 



Most of the genera placed in it are monomyarian ; that is, they 

 have but one adductor muscle, or if two, the anterior one is very 

 small and unimportant. The mantle-edges are entirely open, 

 there being no regularly formed siphons. As a rule, the foot is 

 exceedingly small or quite aborted. 



FAMILY AVICULIDJE 



In this family there is an exception to the rule just stated : the 

 foot is long. There is a byssus, and the gills (by their outer 

 ascending filaments) are concrescent with the mantle. 



The interior of all the Aviculidce is of a highly polished nacre, 

 and some of the larger species have been extensively collected for 

 mother-of-pearl. To the Aviculidce belong the pearl-oysters 

 ( Meleagrina margaritifera). 



A delightful air of romance and adventure surrounds the 

 pearl-fisheries, about which much has been written. The pearl is an 

 abnormal product formed by the deposition of nacre around some 

 foreign object, such as a grain of sand, or usually, it is said 

 an egg which has failed to develop properly. They are seldom 

 found in individuals having favorable conditions of growth, but 

 those which are greatly crowded together and covered with va- 

 rious parasites and growths are most productive of them. The 

 pearl is the same in substance as the nacreous shell of the pearl- 

 oyster, and its iridescence is due to the fine striae of the undulat- 

 ing layers of which it is composed. The pearl as well as the shell 

 is produced by the shell-glands of the mantle. The value of the 



