296 THE BIOLOGY OF MARINE ANIMALS 



excretion takes place across the general body surface. Owing to simple 

 organization no tissue in these animals is far removed from the external 

 medium. Ammonia is the principal end-product of protein catabolism 

 and is readily eliminated by diffusion. Echinoderms and tunicates are 

 other groups lacking excretory organs. 



Most metazoans, however, possess some kind of tubular structures 

 which discharge fluids to the exterior, and on the basis of their morpho- 

 logical appearance such structures are termed excretory organs. Several 

 types can be distinguished, the simplest and most primitive being nephridia. 

 These are found in flatworms, nemertines, certain annelids and cephalo- 

 chordates. A nephridium consists of a hollow flame-cell or solenocyte 

 lying in the parenchyma and a tubule leading from the solenocyte to the 

 exterior (Fig. 7.1). In the intracellular cavity of the solenocyte there are 

 cilia or flagella, the apparent function of which is to drive fluid to the 

 exterior by the creation of a gradient of hydrostatic pressure. In some 

 instances the nephridium may open into the coelom by a ciliated funnel 

 (nephridiostome). 



Another tubular structure encountered in many metazoans is the 

 coelomoduct. This is a duct which opens into the coelom by a ciliated 

 funnel (coelomostome) and leads to the exterior of the body. The nephri- 

 dium is regarded as an excretory organ, whereas the coelomoduct, primi- 

 tively, is a genital duct. In many polychaetes nephridia and coelomoducts 

 become fused in various ways so as to form a conjoint mixonephridium 

 with excretory and genital functions (Fig. 7.1). 



Molluscs and crustaceans possess kidneys of more specialized structure. 

 In gastropods and lamellibranchs there are one or two excretory organs, 

 derived from coelomoducts, which open into the pericardium by ciliated 

 reno-pericardial apertures (Fig. 7.2). The central portion of the tubule is 

 usually enlarged as a renal sac, often very extensive and bearing lamella- 

 tions and diverticula. Distally a renal duct leads to the pallial cavity. In 

 addition there are pericardial glands surrounding the auricles in lamelli- 

 branchs. These structures are believed to move waste material from the 

 haemolymph into the pericardial fluid. The renal apparatus of cephalopods 

 is similar to that of other molluscs, and consists of a multi-chambered 

 renal sac, connected by small apertures with the viscero-pericardial coelom, 

 and opening to the exterior by paired renal papillae. 



The excretory organs of crustaceans consist of two pairs of antennal and 

 maxillary glands, which open at the bases of the corresponding append- 

 ages. Usually only one pair is functional in the adult, and in the Malacos- 

 traca it is the antennal gland. The kidney of decapods arises in an end-sac 

 (coelomic sac), which communicates with an enlargement known as the 

 labyrinth by reason of its convoluted structure. From the latter an excre- 

 tory duct leads to a bladder which opens to the exterior. There is much specific 

 variation in the structure of these organs. The bladder is sometimes greatly 

 enlarged and the excretory duct very short in marine forms (30a) (Fig. 7.3). 



It must be confessed that very little is known for certain about the 



