Immunity of the skin and mucous membranes 405 



innocuous detritus. Sometimes, however, a number of these fine 

 spores manage to perforate the cutaneous investment of the small 

 crustacean ; quite a small opening is made in the chitinous wall, 

 which in itself is a source of no danger. As soon, however, as a spore 

 of the Saprolegnia approaches this opening, it immediately begins to 

 thrust a process through the small lesion, and from that moment the 

 fate of the Daphnia is sealed. Incapable of opposing the slightest [425] 

 phagocytic resistance to the filaments of the fungus, it is invaded 

 throughout by the mycelium and soon dies. 



The integrity of the skin being so important for the preservation 

 of life, a fairly perfect mechanism has been elaborated for the 

 maintenance of this integrity. All animals, no matter what their 

 position in the animal scale, are liable to lesions and wounds of the 

 surface of their bodies. In the Daphniae I have often 1 observed 

 wounds produced by the bites of other aquatic animals. The surface 

 of these wounds soon becomes covered with a rich microbial vegeta- 

 tion. The leucocytes are brought up to the injured point and there 

 produce a protective layer ; but, at the same time, a rapid proliferation 

 of the neighbouring cells of the epidermis takes place; this closes the 

 wound and separates the skin, so reconstituted, from the micro- 

 organisms. Everything resumes its original order and the leucocytes 

 soon disperse, regaining the blood stream. 



These phenomena, which can be readily observed under the 

 microscope in such small and transparent animals as the Daphniae, 

 may serve as the prototype of those of a number of analogous 

 processes in the animal kingdom. The thicker and more solid the 

 cuticular investment, the more fully it guarantees the animal against 

 the penetration of micro-organisms. Cue*not 2 made the observation 

 that Crustacea, furnished with such a hard envelope as is the carapace 

 of the Decapods, are completely defenceless from the moment parasitic 

 micro-organisms make their way into their bodies. These intruders 

 quietly instal themselves in the tissues, without causing the slightest 

 phagocytic reaction, and thus bring about the inevitable death of the 

 host. The protection of the animal in this case is, so to speak, 

 associated with the resistance offered by the carapace. 



Again, in many of the Vertebrata, the skin has a hard, thick 

 sheath, e.g., the scales of fishes and of reptiles. Man, with his 

 supple and not very thick skin, is less well endowed ; this, however, 



1 Vir chow's Archie, Berlin, 1884, Bd. xcvi, S. 192. 

 " Arch, de Biol^ Gand et Leipzig, 1893, t. xm, p. 245. 



