STRUCTURE AND FUNCTION IN INVERTEBRATES 



flame-bulb systems; nephridia (including^ metanephridia), which gencrallv 

 possess cihated coelomic funnels; and Malpighian tubules. Protonephridial 

 systems are commonest among acoelomate and pseudocoelomate Bilateria; 

 they occur in practically all Platyhelminthes, in many Aschelminthes, in 

 Entoprocia, and others; but they are also found in eucoelomate forms, in the 

 larval stages of many mollusks, in annelids, and even among the invertebrate 

 chordates such as Branchiostoma. With such a wide phylogenetic distribution, 

 protonephridial systems exhibit great variability. The chief unifying feature 

 is the presence of flame bulbs, but these may consist of individual cells with 

 single tufts of cilia, groups of cells with numerous ciliary "flames," or single 

 or grouped cells each bearing a single, long flagellum beating in a tube; this 

 last type is generally termed a solenocyte (Fig. 17.6). A primary excretory 

 function has never been conclusively demonstrated for protonephridial sys- 

 tems. The evidence indicates rather that they are involved in maintenance 

 of the water balance of the organism. However, like protozoan contractile 

 vacuoles, it is likely that in eliminating excess water they also flush out wastes 

 in solution. 



Nephridial or metanephridial systems are characteristic of many types of 

 eucoelomate animals. Morphologically, the systems vary considerably from 

 one phylum to another, but they usually exhibit as a common feature ducts 

 with funnels opening into the coelomic cavity or its derivatives. In pelecypod 

 mollusks, for example, there is a single pair of these organs, and the funnels 

 open into the pericardial coelom. In such annelids as the clamworm and the 

 common earthworm, there is a pair of nephridia with individual ducts and 

 external nephridiopores for each of the segmental coelomic cavities (Fig. 

 17.5). In chelicerate and crustacean arthropods, where the coelom has been 

 largely replaced by a hemocoel, the excretory organs (coxal glands and green 

 glands, respectively) are interpreted as remnants of a primitive series of 

 paired nephridial organs; the internal end sac of the crustacean green gland 

 is thought to be a vestige of the ancestral coelomic cavity (see Fig. 15.6, 

 p. 433). The assignment of excretory functions to nephridial systems rests 

 on firm experimental evidence, although details of the processes involved are 

 unknown for many types. In mollusks and annelids the excretory organs are 

 associated not only with the coelomic fluid but also with specific blood vessels. 

 The functions of these organs, in these annelids in which they have been 

 especially studied, appear to involve an initial passage of wastes from the 

 coelomic fluid, followed by selective reabsorption of salts into the blood 

 in the specific vessels associated with the tubular parts of the organs. In 

 crustaceans the functions of the green glands and the nature of the urine, 

 whether concentrated or dilute, appear to be correlated with the environment 

 of the animal. In the fresh-water crayfish, for example, with well-developed 

 tubules, the salt content of the urine is less than that of the blood; the 

 tubules appear to extract salts from the urine, returning them to the blood. 

 In the lobster, however, the urine has the same salt concentration as the 

 blood; it is not modified in passing through the tubular parts, which are verv 



519 



