29 



Figure 24: Frontal section through the pediveliger of Dreissena polymorpha Pall, at level of 



ganglia (from Meisenheimer, 1901). 



bg — byssus gland; eg — cerebral ganglion; es — esophagus; f — foot; gr — gill rudiment; 



hg — hind gut; mc — mantle cavity; pg — pedal ganglion; pig — pleural ganglion; 



rr — renal rudiment; sh — shell; v — velum; vg — visceral ganglion. 



mantle cavity to the visceral ganglia. He considers this band an osphradium — 

 a chemosensory organ that assesses the quality of the water entering the mantle 

 cavity. 



Bayne (1964) has also reported that each larval stage has its own reaction 

 threshold to external stimuli. The nature and intensity of the response are 

 greatly influenced by temperature. The various responses of the larval stages 

 of M edulis to direct light, gravity, and pressure under laboratory conditions 

 are presented in Table 1 (from Bayne, 1976). 



Integument : As in the veliger, the main protective system in the pediveliger 

 is the shell. At this stage the prodissoconch II markedly enlarges, the hinge 

 system develops, the provinculum lengthens, and the number of teeth increase. 

 In addition to the anterior adductor, a posterior adductor develops, which is 

 located behind the visceral ganglion above the hind gut. In O. edulis, the 

 posterior adductor is present even in the veliger (Fujita, 1933). 



Attachment apparatus : In the pediveliger the byssus gland develops rap- 

 idly and begins to function; it forms as a depression in the actoderm along the 



