204 ZOOLOGY. 



homologous with the tubules of the urinary organs in 

 vertebrate embryos, to correspond, in fact, to the prone- 

 phros. The original discoverers of these organs described 

 the internal openings as surrounded by numerous long 

 protoplasmic processes ending in little knobs ; these were 

 called thread-cells. Mr. G-oodrich, having carefully rein- 

 vestigated these organs, found that there were no ccelomic 

 openings, and that the thread-cells were in reality soleno- 

 cytes, the thread being a very fine tubular structure and 

 the knob a nucleated cell. Within each tube is a long 

 vibratile flagellum which extends into the cavity of the 



O v 



organ. The structure is represented diagrammatically in 

 figs. 109 a and 1096. Fig. 109 a shows the entire excre- 

 tory tubule, only moderately magnified; fig. 1096 shows 

 the structure of the solenocytes connected with the blind 

 extremity of one of the branches of the tubule. Although 

 it is not yet known whether these excretory organs are 

 developed from epiblast, the presence of the solenocytes is 

 strong evidence that they are true nephridia. The renal 

 tubules of vertebrates, on the other hand, are not nephri- 

 dia, but coelomic ducts which have taken on excretory 

 functions and have retained to a great degree their original 

 function as generative ducts. The excretory organs of 

 Amphioxus, therefore, are not homologous with those of 

 vertebrates. 



On the other side of the narrow co3lom, just opposite 

 each nephridium, is a little network of blood-vessels, on 

 the course of the branchial vessels just before they enter 

 the dorsal aortae (fig. 104). These little networks have 

 some resemblance to the glomeruli found in the kidney of 

 higher Vertebrates, and are often called by that name ; but 

 as they do not correspond exactly with those structures, it 

 is better to adopt a different name for them, and the term 

 glomus is a convenient variation of glomerulus for this 

 purpose. Apparently water is filtered off from each glomus 

 into the coelom, and excreted by the nephridia. Whether 

 urea is also filtered off from the blood at the same time is 

 difficult to say ; but seeing that the muscles the most 

 active tissues in the body are not supplied with blood 

 but only with lymph, and that this and the crelonaic fluid 



