The Microscope. 



Ill 



ceive the canal connecting the various pouches to be closed, 

 and those leading to the exterior to become specialized, we 



Fig. 3. Diagram of Invertebrate stock: a, anus; c, ccelomatic cavities; d, 

 outline of alimentary tract; k, kidneys ; m, mouth ; n, nervous system; o outlet 

 of kidneys. 



Fig. 4. a, anus; ap, anal pore; b, brain; c, coelom: d, nepbridial duct; g, 

 gill cleft; k, kidney; m, mouth; n, nerve. 



shall obtain the type of the kidneys occuring in the inverte- 

 brate series. 



In the vertebrates the kidneys open into the alimentary 

 tract, and not directly to the exterior. If we close up the ex- 

 ternal openings (cinclides) of the sea-anemone, and allow the 

 longitudinal canal to remain open, portions being specialized 

 as before, the result will be a type found in the vertebrates, the 

 longitudinal canal forming the duct. 



Besides the kidneys the gill clefts of vertebrates are meta- 

 nurically arranged. These, according to Sedgwick, are derived 

 from one of the pouches of the sea-anemone, which retains its 

 connection both with the alimentary tract and with the exterior, 

 but loses that with the longitudinal canal. 



We cannot descend into details, but would refer the reader 

 to Mr. Sedgwick's admirable paper for a full exposition of his 

 views and the arguments in favor thereof. It will be found in 

 the Quarterly Journal of Microscopical Science for January, 

 1884. It is well to say, in passing, that so far as it goes this 

 theory well accords with the teachings of embryology, and, 

 while it has its faults, it explains more facts than any other yet 

 advanced. 



