THE INTERNAL STRUCTURE OF THE OYSTER. 715 



The nervous system of the Oyster is very simple, and, as elsewhere stated, is to some extent 

 degenerate in character. It is composed of a pair of ganglia or knots of nervous matter, which lie 

 just over the gullet, and from these a pair of nervous cords pass backward, one on each side, to 

 join the hinder pair which lie just beneath the adductor muscle. The mantle receives nerve 

 branches from the hindmost ganglia or knots of nervous matter; these, as their centers, control the 

 contraction and elongation of the radiating bundles of muscular fibers, as well as those which lie 

 lengthwise along the margin; the former contract and withdraw the edges of the mantle from the 

 margin of the shell, while the latter in contracting tend to crimp or fold its edges. The tentacles 

 are mainly innervated by fibers emanating from the hindmost ganglia, while the internal organs 

 are innervated from the head or cephalic ganglia. The hind ganglia also preside over the 

 contractions of the great adductor muscle. The nerve threads which radiate outward from it to 

 the tentacles dispatch the warnings when intruders are at hand that it must contract and close 

 the shells. 



211. THE MINUTE ANATOMY OF THE OYSTER. 



There is a spacious segmentation cavity developed in the embryo which becomes the 

 subdivided body-cavity — schizocoel of later stages. Between the ectoderm and endoderm the 

 mesoblastic tissue is developed apparently by proliferation, so that the segmentation or body 

 cavity becomes in part obliterated. The mesoblast of the embryo formed as above stated is the 

 tissue from which the mesenchyme or connective tissue of the adult is developed. The blood 

 channels or canals are developed in the mesenchyme of the adult — mesoblast of the embryo. 

 The large, coarse vesicular connective tissue cells form a sort of trabecular network of pillars and' 

 transverse supports between and around which the sanguineous iluids circulate. The blood 

 channels or canals are developed directly from the spaces between the columns and their 

 conjoining masses of connective tissue cells ; an exception to this is found only in the structure 

 of the anterior and posterior aortse, the heart, and branchiocardiac vessels, which have proper 

 walls lined with endothelial cells. Throughout the greater part of its extent the mesenchymal or 

 connective tissue is spongy, its cells being built around complex anastomosing spaces for the 

 blood. There is, therefore, a true schizoccel developed in the Oyster ; it has been formed as the 

 mesoblastic tissue has grown into the segmentation cavity and subdivided the latter into haemal 

 canals and spaces. The blood cells originate in all probability in the same way. These are 

 amoeboid, colorless, and measure about one three-thousandth of an inch in diameter. The vascular 

 channels have no specialized endothelial walls in the mesenchymal parts of the body. 



The adductor muscle of the shell and the radiating muscular bundles running from the 

 insertion of the former to the edge of the mantle are derived from the mesoblastic cells of the 

 embryo, the observations of Dr. Horst on this point having, I think, completely set at rest what 

 was formerly a matter of theory. The radiating muscular bundles — pallia! muscles — of the adult 

 lie just beneath the epiblast or epithelium on the outer sides of the mantle leaves. These pallial 

 muscles in the embryo are represented by two sets of dorsal and ventral muscular bundles, the 

 functions of which are to retract the velum into which they are inserted. The muscular fibers of 

 the walls of the heart are not striated and decussate in every direction. The inner walls of the 

 heart are crossed in various directions by muscular bands or trabeculse, and a more or less 

 complete muscular septum divides the ventricle in the median line; the heart is, therefore, 

 approximately four-chambered. 



The mesenchymal or mesoblastic tissues comprise the great bulk of the body of the animal, 

 and extend out into and form the greatest proportion of the thickness of the mantle, and also 



