194 BULLETIN OP THE BUREAU OK FISHERIES. 



Jackson (1888) observed that in the oyster this first formed shell was strikingly differ- 

 ent from that which immediately succeeds it and which is retained throughout the rest 

 of its life. It is different in form, composition, and histological structure and covers 

 different organic soft parts. He considered it to be the homologue of the protoconch 

 of cephalous mollusks and the periconch of Dentalium and has named it therefore the 

 prodissoconch (early double shell). For the adult double shell, which is believed to be 

 homologous with the adult single shell of cephalous mollusks, he has suggested the name 

 dissoconch (double shell). The age at which the dissoconch begins to form could not 

 be determined by the author, for it was impossible to keep larvae growing normally in 

 the laboratory for more than five days. Specimens showing the developing dissoconchs 

 had to be collected from the open sea, where it was impossible to estimate their age 

 with any degree of accuracy. 



The velum becomes the organ of locomotion when the prodissoconch incloses the 

 body. It is formed from the apical plate with its encircling zone of cilia and the sur- 

 rounding tissue. It keeps its location in the anterior region, where it grows relatively 

 very large. Retractor muscle fibers, which have their origin in the dorsal region of the 

 valves, are inserted into the velum and provide for the contraction and withdrawal of 

 the organ into the cavity between the valves (fig. 1831, RV, p. 193). When fully pro- 

 truded, the velum projects laterally, considerably beyond the margins of the shell, 

 especially when the valves are nearly closed. Its large, vibrating cilia make it a power- 

 ful locomotor organ, by means of which the larva is able to swim with great rapidity in 

 any direction or to creep on solid surfaces under the water. The velum is also highly 

 sensitive and may be withdrawn with a sudden jerk the instant it is touched by a foreign 



object. 



TRANSITION TO THE ADULT. 



The metamorphosis of the larva into the adult is characterized by the degeneration 

 of the velum, which is the most prominent organ of the trochophore stage, a marked 

 advance in the number and complexity of the internal organs, and a decided change in 

 the external form of the animal. In describing this transition the development of the 

 organs will be considered separately as nearly as possible in the order in which they 

 appear. 



MANTLE. 



The mantle has its origin in the shell gland which first grows to cover the whole dorsal 

 surface of the trochophore larva. This is indicated by the extent of the prodissoconch 

 shell shown in figure 183, e to g (p. 193) . Lateral folds of the shell gland are then formed, 

 which grow downward as the right and left mantle lobes. These develop so rapidly 

 that before the end of the trochophore stage is reached they come to envelop the entire 

 body except when the velum is extended. The result is a decided change in the shape 

 of the animal from a cylindrical to a laterally compressed form (fig. 1S3, e to i, p, 193). 



SHELL- 



The origin and development of the embryonic cuticular shell, or prodissoconch, 

 have been described above. Later the calcareous, prismatic shell, or dissoconch, which 

 is characteristic of the adult mussel, develops. It begins as a limy glandular secretion 

 from the mantle that is deposited in two centers symmetrically placed on the right and 



