254 Dr. T. Williams on the Mechanism of Aquatic 



elaborative reaction of, a second system of cells. These last 

 cells (B, /) are very unlike the former. In the gland of Hclia; 

 aspersa they are best seen. Those which are disposed around 

 the circumference of the space enclosed by the crecum of the 

 gland are the largest, and each of these cells contains a very 

 apparent nucleus which is charged with greenish-yellow granules. 

 As the eye approaches the centre of the caecum, the cells lose 

 their nuclei and become filled only with a mucus-like, pellucid, 

 semifluid substance, which eventually escapes into the excretory 

 channels of the gland. In the gland of Buccinum the dark 

 nuclei of the circumferential cells (B, /) are much less dis- 

 tinct. But the mass of cells by which the caecum is filled 

 presents the same characters as in Helix. It is impossible here 

 to avoid the reflection that the cells are considerably larger in 

 size and more numerous in the glands of an Invertebrated than 

 in those of a Vertebrated animal. In that of the latter, a net- 

 work of blood-vessels accurately fits over and embraces exter- 

 nally the csecal extremities of the gland. In the Invertebrate 

 animal all glands are constructed in this respect on one genei'al 

 type. The blood courses in large streams through the stromatous 

 substance; it never penetrates the hyaline membrane of the 

 follicle. Those parts of the blood which are appointed to nourish 

 the cells upon the agency of which the act of secretion depends, 

 and which cells in their turn furnish the final secreted product, 

 are destined therefore to pass through an external stratum of 

 cells and through the basement or hyaline membrane of the fol- 

 licle before they eventually reach the true secreting cells. This 

 exemplifies the important part taken by cells in the act of 

 secretion. 



Let the physiologist now review the apparatus whose anato- 

 mical constitution has in part been defined. A large excretory 

 duct (fig. 8, B, g), discharging its contents into the rectum, is 

 traceable as other ducts into a constantly diminishing series of 

 ductSj until at length the Florence-flask-shaped terminal vesicles 

 (B, e) are reached. At this point the microscope discloses a 

 complex system of cells and channels for the transmission of 

 fluid, — a machinery in fact which is little inferior in intricacy to 

 that of the glands of the highest animal. It may be reasonably 

 inferred, that such an organized arrangement in a group of 

 animals comparatively inferior must be designed to furnish a 

 product far more important than the " mucosity " as supposed 

 by Cuvier. It is next to impossible that this secretion can be 

 any other than the wnne. But this conclusion should receive 

 all the certainty of an unquestionable demonstration, since, in 

 this case, these bodies in the Gasteropods may be recognized 



