28 



opening at the tip of the foot. Such a gland has been reported for the pedivehger 

 of O. edulis and M. edulis, but is absent in the pediveliger of the scallop 

 Placopecten magellanicus. Hence during creeping the latter larva is not at- 

 tached to the substrate (Gruffydd et al., 1975). Contraction of the posterior part 

 of the foot hauls the entire larva forward. (3) The foot relaxes and the loco- 

 motory cycle is repeated after a few seconds. 



Creeping serves to attach the larva to the substrate. Once the foot is 

 retracted, i.e., loses contact with the substrate, swimming is resumed (Lane and 

 Nott, 1970). 



Nervous System : In the pediveliger stage those elements of the nervous 

 system are most developed which are associated with the fiinctioning of the 

 foot. The pedal ganglia, interconnected with commissures, constitute the ter- 

 minal point of the anterior pedal nerves passing along the tip of the foot, which 

 send sensory impulses ft^om the ciliated sense organs. Besides the cerebral and 

 pedal ganglia, visceral ganglia are also developed, which are situated near the 

 posterior adductor. Rudiments of the pleural ganglia are located near the 

 cerebral ganglion and later merge with the latter in all bivalves except the 

 protobranchia (Figure 24) (Hickman and Gruffydd, 1971; Cranfield, 1973). 



Sense organs : Ciliated sense organs of the pediveliger are represented by 

 the apical plate and sensory cilia situated on the foot, primarily the long 

 mobile cilia on its tip. Sensory cilia also occur in the ftirrow and byssus duct 

 of the foot. 



In many pediveligers there are statocysts and eyes in the mantle cavity. 

 A highly developed nervous system and sense organs are characteristic of 

 planktotrophic larvae, which live for a long time in the plankton. In the 

 planktotrophic pediveliger of O. edulis there are two openings 7—9 |im in 

 diameter along the sides of the base of the foot, which lead into the statocyst 

 cavity. These openings are surrounded by two rings of cilia: short cilia 2—4 |im 

 long and long cilia about 15 |im long (Waller, 1981; see Figure 13). A statolith 

 forms in the cavity of the statocyst. The duct is later blocked and the statocyst 

 becomes a closed vesicle (Figures 25 and 26). In O. edulis, the duct of the 

 statocyst is retained even in the aduU. Here, not far from the base of the foot, 

 there are two eyes in the larvae of many species. Each eye is an ahnost round 

 cup of pigmented epithelium filled with a jellylike substance (Hickman and 

 Gruffydd, 1971). The opening of the cup is covered with a transparent 

 crystalline lens. The size of the eye spot in Mytilus edulis and Modiolus 

 modiolus is 5-10 am (Schweinitz and Lutz, 1976) and in Tiostrea (Ostrea) 

 lutaria, 26-34 |j.m (Chanley and Dinamani, 1980). Nerves to the statocysts and 

 eyes arise from the cerebral ganglion. In the pediveliger of M. edulis, as 

 described by Bayne (1971), a band of ciliated cells passes from the roof of the 



