36 CHORDATE ORGANIZATION n. 9- 



flame-cells do not open internally, but are in close contact with special 

 blood-vessels (glomeruli) whose walls separate the flame-cells from 

 the coelomic epithelium. Assuming that there are 200 of these nephri- 

 dia, each with 500 solenocytes 50 \x long, Goodrich, who has provided 

 the most accurate information about these organs, shows that the total 

 length available for excretion is no less than 5 metres. It is assumed 

 that excretion takes place by diffusion through the flame-cell wall, the 

 liquid being driven down the tube by cilia. Coloured particles injected 

 into the blood-stream are not excreted by the nephridia. 



In development these remarkable organs arise from groups of cells 

 close to the meeting-place of ectoderm and endoderm; almost cer- 

 tainly they are derived from the former. They have no relation what- 

 ever to the mesoderm and this fact alone sufficiently indicates that 

 they are in no way comparable to the pronephros of vertebrates, as is 

 sometimes stated. There is no organ in vertebrates with which they 

 can be compared, nor is there any trace in amphioxus of organs com- 

 parable to the vertebrate kidney system. In fact we have here a 

 remarkable case of an isolated feature; evidently separate items of the 

 genotype may vary independently, and the whole bodily organization 

 does not necessarily change together. 



The brown funnels are blind sacs at the front of the atrium, 

 invaginating into the epibranchial coelom. They are probably receptor 

 organs. Some parts of the atrial wall may perform excretory functions. 

 Masses of cells in the atrial floor, the atrial glands, contain granules 

 that may be excretory but may have been taken up from the food 

 current. 



In the gonads, especially the testes, there are large yellow masses, 

 containing uric acid, which are extruded with the gametes. 



10. Nervous system 



Amphioxus possesses a hollow dorsal nerve-cord similar to that of 

 vertebrates. Though this is somewhat modified at the front end, it is 

 not there enlarged into an elaborate brain. The nervous system is con- 

 nected with the periphery by a remarkably simple set of nerve-roots, 

 a dorsal and a ventral on each side in each segment. The roots do not 

 join (Fig. 13): the ventral roots lie opposite the myotomes, to which 

 they carry motor-fibres, and these end on the muscle-fibres with 

 motor end-plates. The dorsal root runs out between the myotomes 

 and carries all the afferent fibres of the segment and motor-fibres for 

 the non-myotomal muscles of the ventral part of the body. This is the 

 fundamental pattern of the roots in all vertebrates. 



