256 INVERTEBRATE PHYSIOLOGY 



Arthropods 



If we examine very briefly the origin and structure of the excretory 

 organ of Crustacea, as representative of many arthropods, we find the 

 origin to be from the primitive coelomoduct (Goodrich, 1945). It may 

 consist of several parts, of which the first is a thin-walled end sac some- 

 times closed by a special sphincter or valve. Generally the thin, simple epi- 

 theHum of this sac with its outer basement membrane is held open by 

 attachment to surrounding structures by strands of connective tissue. 

 This sac may offer an opportunity for filtration through the walls from the 

 surrounding blood of the hemocoel. Probably of more importance for filtra- 

 tion is the direct arterial blood supply found in many species. Parry ( 1955) 

 has injected and dissected the blood supply to the kidney in the prawn 

 Palaemonetes varians and finds : "the main branch of the antennary artery 

 on either side of the thorax leads directly to the end sac, where it suddenly 

 splits up into numerous fine vessels which are lost in the walls of the end 

 sac. Neither the labyrinth nor any other part of the gland appears to have 

 any (arterial) direct blood supply ..." The end sac communicates through 

 the valve with a canal of greater or lesser length depending upon the 

 species, a canal lined with thicker cells by means of which, it may be in- 

 ferred, active changes may be brought about in the character of the urine. 



The beginning of balanced studies to test for filtration as a factor in 

 arthropod excretion may be said to have been made by Picken (1936). 

 Measuring colloid osmotic pressure of urine and blood and the hydrostatic 

 pressure of the body fluids of Carcinns macnas, Potamohius fluviatilis, and 

 Peripatopsis sedgwicki, he concluded that a filtration pressure exceeded 

 the colloid osmotic pressure in each of these animals so that filtration would 

 naturally occur. It may not be safe to argue entirely from the hydrostatic 

 pressure of the body fluid, but we may guess that the pressure in the 

 arteries supplying the kidneys is even greater than this measured hydro- 

 static pressure. It seems at least likely that filtration may occur. 



Some more recent investigations have produced evidence in favor of a 

 process of filtration in the kidneys of other arthropods. It should be stated 

 at once, however, that, until some further experiments are performed on 

 the crayfish, all evidences for filtration in the fresh-water arthropods must 

 be regarded with some skepticism. This is so because in the crayfish alone, 

 among all the animals so far investigated, there appears to be evidence of 

 the secretion of inulin. Maluf (1940, 1941a) interprets his evidence to in- 

 dicate that all of the components, including water and inulin, of the urine 

 of the crayfish are produced by secretion. 



In a lengthy report Maluf (1941b) has described the handling of inulin, 

 xylose, creatinine, and certain dyes by the antennal glands of large healthy 



