19 



rotating in the process like a flying cannonball; the veliger of Argopecten 

 irradians exemplifies this type of locomotion. The nature and speed of veliger 

 locomotion depend on change in salinity, temperature, and pressure 

 (Mileikovsky, 1973; Cragg and Gruffydd, 1975; Hidu and Haskin, 1978; 

 Cragg,1980). 



The veliger of Mercenaria mercenaria ascends at a speed of 7—80 cm/min 

 (Turner and George, 1955; Carriker, 1961), the late veliger of Crassostrea 

 virginica at a maximum speed of 60 cm/min (Wood and Hargin, 1971), and 

 the veliger of Lyrodus pedicellatus at a speed of 45 cm/min (Isham and 

 Tiemey, 1953, cited by Mileikovsky, 1973). 



Nervous System : Under the apical plate of the velum lies the cerebral 

 ganglion connected with the anterior part of the plate. Initially, the cerebral 

 ganglion is a single structure that later becomes bilobate. Numerous nuclei and 

 nerve cell fibers are visible in sections. In the middle part of the ventral side 

 there are two large pedal ganglia, which later join through connectives with 

 the cerebral ganglion (Figure 16). Development of the pedal ganglia precede 

 that of the foot (Meisenheimer, 1901; Cranfield, 1973). 



Sense organs : Considerable development of the nervous system and diver- 

 sity and degree of development of the sense organs are associated with the high 

 locomotor activity of the veliger. The most noteworthy sense organ of the 

 veliger is the apical plate, which may be drawn in, forming an apical pit. The 

 diameter of the pit in O. edulis is 10-15 jim. In its anterior part lie 20-100 cilia, 

 6-8 |i,m long, which perform a sensory function (Waller, 1981). In Teredo 

 navalis the apical plate bears only three complex cilia. As the apical plate grows, 

 the cilia shift to its posterior half (Culliney, 1975). A sensory fiinction is also 

 performed, evidently, by the cilia of the intemal preoral band. These are small 

 cilia, about 20 |im long, arranged irregularly. In the veliger ciliary bands and the 

 ciliated field generally fimction in locomotion and reception. 



Integument : Protection of the veliger from unfavorable extemal influences 

 is done by the cells of the outer epithelium; a frnn contact between them 

 ensures some protection of the larva. According to Waller (1981), the cells of 

 the outer epithelium are covered with mucus; apparently, the secretion pro- 

 duced by these epithelial cells also has a protective ftinction. However, these 

 adaptations are accessory in character relative to the prime protective struc- 

 ture — ^the shell. Like other organs of the larva, the shell simultaneously per- 

 forms several ftinctions: protective, supportive for muscle attachment, and jet- 

 directing; however, the primary, and often basic fiinction is protection. The 

 shell, as already mentioned, is a product of the activity of secretory cells of 

 the shell gland. The peripheral cells of the shell gland located along its narrow 

 opening secrete the primary unpaired organic periostracum (Kniprath, 1979), 



