THE LIFE-HISTOBY OF NUCULA DELPHINODONTA. 377 



otocystic canals ofYoldia limatula. The canals are short 

 and do not reach the surface of the foot. Frequently a number 

 of closed pouches are connected with the end of each otocystic 

 canal by a strand of fibrous material (Text-fig. X). Stempell 

 reports that the otocystic canals of Leda pella are very 

 rudimentary. He has never been able to trace them with 

 certainty to the surface skin (17). The conditions illustrated 

 by these two forms can easily be explained as the result of 

 degeneration, but there is no direct evidence that this is the 

 case. If the canals are not remnants of the original invagiua- 

 tions, the imperfect canals may be structures that have never 

 been perfect canals, or they may indicate the degeneration 

 of canals that have at some time been perfect. 



The nerve-supply of the otocysts has been discussed in 

 connection with the nervous system. It seems possible that 



Text-fig. X. — Otocystof Yoldia limatula. cpc. Cerebro-pedal commissure. 

 oc. Otocystic canal. oL Otolith. OJi. Otocystic nerve, op. Otocystic 

 pouch, ol. Otocyst. 



the dorsal roots of the cerebro-pedal commissures may be the 

 central ends of the otocystic nerves. 



Muscular System. 



For convenience in treating the subject the muscles may 

 be grouped into those concerned in shutting the shell, in 

 moving the foot, in propelling blood, in retracting the 

 margins of the mantle, in retracting the palp appendages, 

 and in raising the gills. Beside these muscles, there are 

 many scattered and interlacing fibres that are concerned in 

 making many of the movements. 



VOL. 44, PART 3, NEW SERIES. B B 



