Microns 



Figure 261. — Transverse section of the cerebro-visecral 

 connective of C. virginica. Large aggregation of nerve 

 cells on the periphery. Kidney tubules on extreme left. 

 Kahle, hematoxylin-eosin. 



nerve cells are predominantly at the periphery. 

 Along its entire course the nerve gives rise to 

 numerous branches which at regular intervals enter 

 the tentacles and terminate at their surface in a 

 number of small fibers which can be seen in gold- 

 impregnated preparations (fig. 86, ch. V). 



Small peripheral nerves such as the branchial 

 nerves (fig. 263) and the radial nerves of the mantle 

 (fig. 264) are made of bundles of fibers with occa- 

 sional small nerve cells between them . Tlie sheath 

 of these nerves consists of a narrow layer of 

 spindle-shaped connective tissue cells. A well- 

 developed nerve net, visible in gold-impregnated 

 sections (fig. 87, ch. V), is found over the entke 

 mantle. 



The anatomical and Iiistological picture de- 

 scribes an intercomnmnication system which 

 connects all the organs and parts of the oyster. 

 Stimulus received, for instance, at the dorsoan- 

 terior part of the mantle and transmitted to the 

 cerebral ganglia may reach the visceral ganglion 

 either througli the cerebro-visceral connective or 

 directly via the numerous nerve branches which 

 extend from the edge of the mantle and are 

 connectctl by the circumpallial nerve. Stinmli 

 transmitted to cephalic ganglia may reach the 

 visceral ganglion through one of the connectives 



and vice versa. Thus, in spite of the absence of 

 a central nervous system, the nervous reactions 

 of the oyster are well integrated. 



The major movements of the oyster are hmited 

 to the contraction and relaxation of the adductor 

 muscle. Muscular contraction may be provoked 

 either by the stimulation of the receptors of the 

 tentacles and mantle or by impulses which origin- 

 ate from the internal organs. An example of 

 the latter type of stimulation is found in the 

 spawning reaction of the female. It consists of a 

 series of rhythmic contractions of the adductor 

 muscle associated with the release and dispersal 

 of ova by the sexually mature oyster but in the 

 immature specimen it cannot be induced by drugs, 

 mechanical, electric, or thermal stimuli. The 

 reaction is fully discussed in chapter VIII, page 172. 



THE PALLIAL ORGAN 



The only sense organs of the oyster are the 

 tentacles of the mantle edge and the palhal (also 

 called abdominal) organ. The structure and in- 

 nervation of the tentacles have been already 

 discussed in chapter V, page 85. The pallial 

 organ is a very small structure attached to the an- 

 terior side of the adductor muscle inside the ex- 

 halant chamber of the gills. In order to see the 

 organ the wall of the cloaca and of the epibranchial 

 chamber must be dissected and the two sides 

 drawn apart in the manner shown in figure 236 

 in chapter XI, page 259. The pallial organ is 

 a small, colorless protuberance marked on the 

 drawing by the letters pal. or. 



The structure was discovered by Thiele (1889) 

 in a number of bivalves including 0. edulis. In 

 tlie European oyster the organ was described as a 

 comma-shaped protuberance ("kommaformige 

 Erhebung") with the pointed end turned to the 

 right and the concave side oriented toward the 

 posterior end. The organ on the right side is well 

 developed but on the left side is small and degen- 

 erate. The description and illustration published 

 by Thiele apply to C. virginica. In this species 

 the pallial organ on the right side is also better 

 developed while on the left side it is much smaller 

 and frequently absent. A similar condition is 

 found in 0. cucullata (Awati and Rai, 1931). 



The structure of the pallial organ of C. pirginica 

 is revealed in a series of sagittal sections (fig. 265). 

 The rounded surface of the organ is covered by 

 elongated epithelial cells with hairlike cilia which 

 are longer than the cell bodies. This type of cell 



THE NERVOUS SYSTEM 



289 



