132 DEVELOPMENT. 



ming disk, fringed with long cilia^ and armed with a slender 

 tentacular filament (Jiagellum). At a later period this disk 

 disappears progressively as the labial palpi are developed ; and 

 thej^ acquire a foot, and with it the power of spinning a byssus. 

 They now have a pair of eyes situated near the labial tentacles 

 which are lost at a further stage, or replaced by numerous rudi- 

 mentary organs placed more favorably for vision, on the border 

 of the mantle. 



The shells of theembrj^onic Uniones and Anodons (xx, 56, 51), 

 are so dissimilar to those of the adults, that they were at one 

 time considered to be parasites (Glochidium). The valves of 

 the shell are usually triangular, with a flattened hinge-line and 

 they frequentl}'^ have incurved hooks on the basal margin, by the 

 help of which the larvae, after they leave the parent, attach 

 themselves to its shell, or to fishes and other floating bodies. In 

 this position they undergo metamorphosis, and eventually fall 

 oflT and sink to the bottom as minute fresh-water Mussels. The 

 3^oung of some species are attached by a filament, and Dr. Lea 

 believes that they are always unimusculose (monomyary), the 

 muscle dividing during metamorphosis.* 



The development of the oyster (xx, 60-63), presents some 

 interesting particulars. The embryo, at first spherical, becomes 

 cordiform, then two or three vibratile cilia appear on its surface, 

 and a transparent tract indicates the place of the hinge — in the 

 vicinity of which is formed a calcareous deposit, the first indica- 

 tion of the valves. The velum soon greatly develops, so that 

 its complete retraction is not possible. By the aid of this organ 

 the animal swims rapidly, but without knocking the surrounding 

 embryos. The mouth is encircled by a crown of cilia. The 

 velum finally disappears, or is possibly transformed into the 

 labial palpi. Finally the shell, until now equivalve and free 

 becomes fixed for life, and from this moment commences the 

 normal life of this mollusk.f 



In Pisidium,the oral extremity of the embryo is velated. On 

 its dorsal face the tegument is raised to form a mantle, at the 

 centre of which is produced by invagination a small sac or precon- 

 chylian gland, having a temporary existence (Lankester). This 

 gland occupies the place of the ligament ; the shell is afterwards 

 developed on the surface of the mantle under the form of a 



* Lea, Obs. on Unio, vi, 43. 



f The eggs of the American oyster (0. Virginica), are fertilized outside 

 the body of the parent, and the young swim at large during the period in 

 which the fry of the European species ( 0. edulis) are sheltered in the 

 mantle of the parent. Segmentation of the vitellus in the first named is, 

 according to Brooks, completed in about two hours.— J^to. Jour. Science, 

 xviii, 425, 1879. 



