716 NATURAL HISTORY OF AQUATIC ANIMALS. 



down into the branchial sacs between their epiblastic or epithelial, ciliated, external walls. It 

 also forms the principal bulk of the thick vertical, transverse septa which subdivide the branchial 

 pouches internally, and forms likewise the bulk of the branchial filaments themselves. These 

 latter are numerous and give the surface of the gills their- furrowed or plaited appearance. The 

 individual plaits or ridges seen in section are found to be quite complex and to be themselves 

 eompoundly ribbed and to have chitinous rods embedded in their substance just beneath the 

 external epithelium. These rods run lengthwise through the substance of the branchial riblets. 

 The branchial capillaries are excavated in the mesenchymal or connective tissue of the branchial 

 filaments or tentacles, between which there are numerous openings or ostia for the passage of the 

 water from the inferior portion of the pallial chamber into the gill cavities in order to effect 

 respiration. It is difficult, however, to make this arrangement understood without the aid of 

 figures. 



The mesenchyme also gives support to all of the visceral structures, the ultimate secretory 

 follicles or saccules of the liver being imbedded and supported by it. The same is true of the 

 generative structures and the intestine. No portion of the walls of the stomach, oesophagus, or 

 hepatic ducts can be found the walls of which do not lie directly in contact with this mesenchymal 

 or mesoblastic tissue. It also extends out into and forms the greater proportion of the substance 

 of the palps or lips of the Oyster, and is very spongy and highly vascular in this region. The 

 internal or oral surface only of the palps or lips are closely plaited with numerous folds of ciliated 

 epithelium. These folds may number from one hundred and twenty-five or more. The surface of 

 the palps in the immediate vicinity of the mouth is not plaited or folded. 



The mesenchymal cells are much larger than either the epithelial or endothelial cells, and will 

 average one five-hundredth of an inch in diameter. They inclose in all cases, both in winter and 

 summer, a large, irregular nucleus from which a complex network of intracellular granular fibrils 

 radiate in all directions through the enveloping cellular substance. At one side of the nucleus 

 there are always one or more accessory bodies, perfectly globular, which complicate the character 

 of the nucleus in a singular manner. These vesicular, very hygroscopic, meseuchymal or connective 

 tissue elements are not fat-cells, as has been erroneously supposed by Brooks. Their nuclei are 

 invariably central and not parietal in position, as in fat-cells. These cells are probably very 

 hygroscopic, as would appear judging from their singular appearance under the microscope. They 

 appear to be widely distributed in the molluscous invertebrates; they were originally named 

 "vesicular connective tissue cells "by the histologist Schaefer. An Oyster may in the summer 

 season absorb water and swell up so as to fill up almost the whole cavity of the shell, and when 

 opened it may lose so much blood and water in the course of half an hour that it will have shrunk 

 to one-tenth of its original bulk. This is a common occurrence, and is explained by the prob- 

 able hygroscopic character of the connective tissue cells and the spongy nature of the whole 

 mesenchyme which consists of these elements. This also explains why it is that Oysters may be 

 much swollen in a short time by osmotic action, when immersed in water of a less specific gravity 

 than the sea-water from which they were first taken. The process has nothing in common with 

 what might be called fattening, as we shall see hereafter. 



There is an apparent atrophy or wasting away of the mesenchyme of the body-mass and 

 mantle during the spawning season, with a great concomitant development of the reproductive 

 follicles or tubules. In winter the reproductive follicles atrophy, when the mesenchyme again 

 increases in bulk in the body-mass and mantle. It also undergoes another remarkable series 

 of changes corresponding to summer and winter. In summer it acquires an almost glass-like 

 transparency, so that the mantle, palps, and superficial portions overlying the viscera become 



