KIDNEYS. 425 



cance. The true body-cavity {Lit) is filled with lymph, 

 its inner wall being clothed by the intestinal-fibrous layer, 

 its outer wall by the skin-fibrous layer. The gill-cavity {A) 

 is, on the contrary, filled with water, and its whole wall is 

 clothed by the skin-sensory layer. The latter envelopes the 

 outer surface of the two large, lateral gill-roofs, the lateral 

 processes from the body- wall, which grow together below 

 round the original ventral side, and unite in the central line 

 (in the ventral seam or raphe. Fig. 152, R). 



On each side of this ventral seam, on the inner surface of 

 the gill-roofs, directly in front of the gill-pore {"porus 

 hranchialis), and over the ventral muscles (J/) and between 

 the sexual glands (G), lie the kidneys of the Amphioxus. 

 These urinary glands are present in the simplest form, as 

 glandular epithelial swellings of the skin-sensory layer. 

 The epithelial cells of these are distinguished by peculiar 

 size and nature, and contain crystalline deposits. As we 

 regard the primitive kidneys of other Vertebrates also as 

 originally skin-glands, and as we derive them from the skin- 

 sensory layer, it is very interesting to find these organs 

 permanently retained in the Lancelet as skin-glands. 



The sexual organs also appear in a perfectly simple 

 form. On both sides of the gill-intestine, in the central part 

 of the gill-cavity, lie from twenty to thirty small elliptical or 

 roundly four-cornered sacs, which can easily be seen by the 

 naked eye from withgut, through the thin transparent wall 

 of the body. In the female, these little sacs are the ovaries, 

 and contain numbers of simple egg-cells (Plate X. Fig. 13, e). 

 In the male, these are replaced by the testes, heaps of 

 smaller cells, which change into movable whip-cells (sperm- 

 cells). Both kinds of sacs lie within on the inner wall of 



