772 BULLETIN OF THE BUREAU OF FISHERIES. 



The amount of water passing through the bottom of the filter does not 

 create an appreciable outward current, and, at any rate, the fry are held above 

 the bottom by the upward trend of the current created by the propellers. Two 

 or three cars of this type have been operated for periods of four to ten weeks at 

 a time. Several varieties of very young fishes and larval invertebrates have 

 been reared with highly satisfactory results. Among the many hundreds or 

 thousands of animals only three or four dead specimens of any kind have been 

 observed. 



Canvas lining for boxes. — A further modification of this method has been 

 adopted in order to prevent the escape of certain very small animals like crabs, 

 which seek out and crawl into very narrow cracks in the wood. It consists of 

 putting into the box a large canvas bag as a sort of lining and arranging the 

 filter pump as usual (fig. i6, pi. xcvii). This apparatus has also proved 

 satisfactory. 



POSSIBILITY OF VARIATION. 



So detailed a description of the apparatus as at present installed and in 

 use might without a further word leave the impression that this apparatus alone 

 fulfills the requirements of this general method of fish culture. On the con- 

 trary, there is hardly a feature of the whole outfit that has not been represented, 

 at one time or another during our experiments, by other materials or other 

 forms. The present boxes, for example, have replaced bags of canvas and of 

 scrim and bobbinet, not because the latter failed to give good results, but because 

 they were less durable and otherwise objectionable. Three forms of power 

 transmission have been operated successfully during the development of the 

 plant. It is obvious that the gasoline engine might under other circumstances 

 properly give place to a different kind of motive power, such as steam or hot-air 

 engines or electric, spring, weight, or water motors. For use in small experi- 

 ments weight or spring motors, properly governed for speed, have much to 

 recommend them, for individual cars could be independently operated in various 

 localities without the inevitable expense and annoyances of running the engine 

 and the apparatus for power transmission. 



PRECAUTIONS. 



There are, moreover, precautions to be taken in the construction of the 

 cars and other devices. New wood, especially pine, and certain metals, par- 

 ticularly copper and galvanized iron, which are frequently used as screens, are 

 apt to injure, and often prove fatal to young animals even when under other 

 circumstances the circulation through the car would be ample. A very striking 

 instance of the effect of small quantities of copper and zinc-plated screening 

 was furnished in an experiment made a year ago at our station by Dr. V. E. 



