42 NATHAN FASTEN 



this cup there was an opening covered by a tightly fitting screw- 

 cap. By unscrewing this cap, each haul could easily be trans- 

 ferred from the cup to another receptacle without losing any 

 of the gathered organisms. Generally, each catch was strained 

 through a fine linen plankton sieve, in order to condense the 

 haul for examination. By means of a pipette the copepods 

 were segregated from the other organisms, and introduced into 

 a clean dish of water, where they were allowed to remain until 

 needed. 



IV. MOVEMENTS OF THE LARVAE UNDER NORMAL CONDITIONS 



In the free living stage, the copepod swims about actively, 

 with a snappy, dart-like spiral motion. Its mouth parts are, at 

 the same time, moving incessantly, always ready to grasp their 

 host and thereby insure the further development of the organ- 

 ism. Movement through the water is accomplished by the two 

 biramous swimming feet on the thorax. In their normal resting 

 attitude, the feet are held in an oblique position, with their 

 setae pointed toward the head of the organism. The abdom- 

 inal portion of the copepod is somewhat bent, at an angle of 

 about 2o° with the long axis of the body. 



Motion is produced by the contraction of the powerful dermal 

 muscles, which cause the swimming feet to dart backwards, 

 thus shooting the organism ahead. The copepod, in its motion, 

 passes through the same angle as that which the swimming feet 

 traversed in the act of propelling the body. The actual distance 

 covered by the copepod through one stroke of its feet is about 

 an inch. Further motion is accomplished by a repetition of 

 the same process; the animal thus travels in a snappy, spiral 

 path (figure 5). When motion ceases, Lernaeopoda usually 

 takes an upright position, with its long axis almost perpen- 

 dicular to the surface of the water. Soon it turns upside down, 

 and slowly begins to sink to the bottom, w r here it may assume 

 a ventral or a dorsal position, depending entirely on which 

 side of the body strikes the bottom first. 



Oftentimes the copepod is found adhering to the side of the 

 glass dish nearest the window, or it may sometimes suspend 

 itself on the surface film of the water. In such cases, the larva 

 maintains an upright position, while the first antennae are 

 stretched out horizontally, making an angle of almost 90 ° with 



