STRUCTURE AND FUNCTION IN INVERTEBRATES 



wastes are collected from the hemocoelic blood into the tubules, where ihey 

 are concentrated by reabsorption of water and passed into the hind-gut (see 

 Fig. 15.19, p. 450). Here further amounts of water are extracted, and the 

 residue is finally eliminated from the body as a relatively dry, crystalline mass. 



Still another mechanism for the removal of wastes from the body fluids is 

 widely distributed among invertebrates, appearing in many forms as the only 

 excretory pathway other than simple diffusion, and in others as an accessory 

 to one of the systems just discussed. This involves the activities of numerous 

 fixed or amoeboid cells, of mesodermal origin, which accumulate wastes from 

 the blood or coelomic fluid and store them as granules or spherules within 

 their cvtoplasm. This storage may be temporary or permanent. In echinoderms 

 amoeboid cells of the coelomic fluid accumulate wastes and engulf foreign 

 particles, and the cells containing these accumulations are eliminated from 

 the body by passing through the walls of the dermal branchiae or "water 

 lungs" into the sea water. Cells in other locations appear to store 

 isolated wastes throughout the life of the echinoderm. In insects the cells 

 of the fat body and other tissues function similarly in the accumulation of 

 wastes; this is particularly notable during the pupal stage, when elimination 

 of nitrogenous excreta from the body is impossible. The activities of all 

 these waste-storing cells, known generally as athrocytes, help prevent wastes 

 from reaching toxic concentrations in the body fluids. From the functional 

 standpoint it is not necessary that the wastes be completely eliminated from 

 the body; they can be as eflfectively isolated by sequestration in certain cells 

 where they are held in "dead storage." 



Broadly comparative studies make it clear that the nephridial excretory 

 systems of eucoelomate invertebrates have developed primarily in connection 

 with the coelomic cavities, and it is probable that their original functions 

 involved the elimination of nitrogenous wastes dissolved in the coelomic 

 fluid. In many of these forms a copious blood supply to the nephridia has 

 been established through the vascular system, indicating a possible shift of 

 the pathway of excretion from the coelomic fluid to the cirulating blood. A 

 parallel development of excretory mechanisms occurs in the vertebrate 

 phylogenetic series and in the ontogeny of the higher vertebrates. Here the 

 earliest excretory organs develop in connection with the coelom, which they 

 drain through ciliated funnels; these primitive relationships are subsequently 

 replaced by vascular connections, which make possible the extraction of 

 wastes from the circulating blood. 



Systems Related to Responsiveness and Integration 



One of the characteristics of living things is responsiveness, the capacity 

 to react to changes in the environment. The behavior of such simple animals 

 as the amoeba is based on, and limited by, the inherent reactivity or 

 responsiveness- of the single cell. Such primitive behavior patterns would 



521 



