262 ZOOLOGY FOR MEDICAL STUDENTS chap. 



Laboratory specimens are best fed with cultures of green flagellates 

 as such specimens become especially transparent and the alimentary 

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

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

 as is 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 are at first simple and pocket-like they become subdivided up so 

 as to form in the adult a complex mass of blindly ending tubes. 



The haemocoehc nature of the body-cavity is readily recognizable 

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

 its spaces 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 

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

 producing fungus Monospora swallowed by Daphnia perforate the wall 

 of its alimentary canal but on reaching the surrounding haemocoele 

 are at once attacked, ingested, and destroyed by the blood-corpuscles 

 (phagocytes). 



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 

 beneath the surface, are conspicuous and show the crystalline cones 

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

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

 the tip of the supra-oesophageal ganglion or brain. 



The ovary is an elongated organ lying alongside the alimentary 

 canal 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 

 projections from the dorsal surface of the abdomen. 



The reproductive phenomena are of special interest. During the 

 greater part of the summer season male individuals are very rare and 

 the females reproduce parthenogenetically. At seasons of the year, 

 however, when climatic conditions are liable to become unfavourable — 

 in autumn when the cold weather approaches and during the early 

 summer when pools are liable to dry up — males (distinguishable by their 

 smaller size, their larger first antennae, and their more rapid and less 

 jerky movements) appear in numbers and the females produce so-called 



