ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



ry specimens are best fed with cultures of green flagellates 



irh specimens become especially transparent and the alimentary 



I stands out distinctly with its green contents. The chief point of 



interest about the alimentary canal is that its digestive glands, of which 



usually the case in Crustacea there are a pair, are in the form of 



simple pocket-like outgrowths of the enteric wall. In such a crustacean 



as a Crayfish or Lobster on the other hand although the corresponding 



glands an- at first simple and pocket-like they become subdivided up so 



i -inn in the adult a complex mass of blindly ending tubes. 

 Tin- haemocoelic nature of the body-cavity is readily recognizable 

 undiT the microscope as the blood-corpuscles can be seen driven through 

 its spares by the beating of the heart. These creatures are of interest 

 in connexion with the history of medical science for it was by observa- 

 tions upon their blood-corpuscles that some of the first steps were made 

 in the scientific investigation of the process of inflammation which plays 

 such a great part in the process of healing by the Russian zoologist 

 MeK Imikoff. He observed how the sharp needle-like spores of a disease- 

 pit Blueing fungus Monospora swallowed by Daphnia perforate the wall 

 of its alimentary canal but on reaching the surrounding haemocoele 

 it once attacked, ingested,, and destroyed by the blood-corpuscles 



The nervous system is of the normal arthropodan type although only 

 the head portion is clearly visible in the specimen when viewed as a 

 whole. The: radiate eyes, which have become fused together and sunk 

 h-Meat h the surface, are conspicuous and show the crystalline cones 

 very ( learly under the microscope. In addition to them there is a 

 pie unpaired eye recognizable as a small speck of black pigment at 

 tin- tip of the supra-oesophageal ganglion or brain. 



The ovary is an elongated organ lying alongside the alimentary 



I and opening on the side of the abdomen into the space between it 



and the carapace. The eggs when laid pass into this space and remain 



there during their development, being kept from falling out by spike-like 



tions from the dorsal surface of the abdomen. 



reproductive phenomena are of special interest. During the 



i the summer season male individuals are very rare and 



the lem.ile^ reproduce parthenogenetically. At seasons of the year. 



r. "lien climatic conditions are liable to become unfavourable 



"> autumn when the cold weather approaches and during the early 



pooh are liable to dry up males (distinguishable by their 



-'<. their larger first antennae, and their more rapid and less 



ppcar in numbers and tin- females produce so-called 



