. The biology of marine animals. Marine animals; Physiology, Comparative. 262 THE BIOLOGY OF MARINE ANIMALS role amoebocytes play in the digestive processes of lamellibranchs by permitting the utilization of diatoms and other food particles which are too large to gain access to the tubules of the digestive diverticula. Extracellular digestion is well de- veloped in echinoderms, but there is also some evidence for the co- existence of intracellular digestive mechanisms. Amoebocytes in the gut epithelium and lumen of echinoids and holothurians participate in di- gestion to greater or lesser degr
. The biology of marine animals. Marine animals; Physiology, Comparative. 262 THE BIOLOGY OF MARINE ANIMALS role amoebocytes play in the digestive processes of lamellibranchs by permitting the utilization of diatoms and other food particles which are too large to gain access to the tubules of the digestive diverticula. Extracellular digestion is well de- veloped in echinoderms, but there is also some evidence for the co- existence of intracellular digestive mechanisms. Amoebocytes in the gut epithelium and lumen of echinoids and holothurians participate in di- gestion to greater or lesser degree by ingesting particles and absorbing dissolved nutriment. Phagocytes in the pyloric and intestinal caeca of asteroids have a similar role (62).. Fig. Section through a Liver Tubule of a Shtpworm, Showing Phagocytic Cells Containing Par- ticles of Wood (after Potts, 1923.) Extracellular Digestion Intracellular digestion is the more primitive mechanism from which ex- tracellular digestion has developed as a specialization, notably in the annelids, Crustacea, cephalopods and chordates, all groups containing active forms. Extracellular digestion has certain apparent advantages in that it breaks down food substances and eliminates indigestible material rapidly and also permits a compact and reduced alimentary canal. Various specializations of structure and function have appeared in conjunction with the secretion of digestive enzymes into the gut lumen. There is regional differentiation for secretion of enzymes, absorption of digestive products and elimination of faeces. Special digestive enzymes are developed to meet particular needs, secretion is regulated and the enzymatic environment is controlled with apparent nicety. Digestive enzymes are secreted by unicellular glands lining the digestive tract or localized in special diverticula and by compound glands of many sorts. In coelenterates secretory cells are dispersed through the endoderm in hydrozoa, localized in gastric f
Size: 1551px × 1610px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, booksubjectmarineanimals, booksubjectphysiology
