OF MASSACHUSETTS. 55 



(Figs. 55-57). Before starting to move the scallop projects its foot, 

 waving it several times around, as if to reconnoiter. Then, suddenly 

 becoming bold it stretches out this organ in a decided manner. The 

 foot is elongated about the length of the body by the contraction of 

 the circular muscles, which is called a " thinning wave " by the Ger- 

 man. The free end is firmly set by a sucker-like arrangement, and 

 a "thickening wave," caused by the contraction of the longitudinal 

 muscles, passes toward the shell. The foot movement is roughly com- 

 parable to the creeping of an earthworm. The shortening of the foot 

 jerks the shell forward, the movement being strengthened by the clap- 

 ping of the valves, which send out a current of water posteriorly from 

 the pseudo-siphon, as is indicated by the moving specks of dirt in the 

 water. The valves shut when the longitudinal muscles contract and 

 open with the contraction of the circular muscles, giving a jerky motion 

 to the crawling. 



The scallop may change its direction in crawling by setting the tip 

 of the foot to either side of the line of motion. When the foot contracts 

 the shell is swung around very effectively. As the animal has never 

 been observed to crawl backward, a frequent maneuver with young clams 

 and quahaugs of this age, it probably reverses its direction by a series 

 of these movements. The animal often changes its base of crawling 

 from the face of the right valve to the free edges of both valves. In 

 this case the valves are so tilted that the posterior portion of the free 

 edge is uppermost, thus making the anterior posterior axis perpendicular 

 to the surface on which it crawls. (The above observations were made 

 on 1 to 2 millimeter scallops.) 



An early dissoconch scallop (phase 4) was observed to take a peculiar 

 position on the bottom of a watch glass. It raised itself on to the edge 

 of the shell with the anterior end high in the water, the foot extended, 

 waving back and forth. Whether it did this by other aid than the lash- 

 ing of its foot could not be ascertained, but it gracefully rose on edge. 

 A similar maneuver was observed in a l^-millimeter scallop. Evidently 

 this habit is useful in turning over when the young scallop finds itself 

 on the wrong valve. 



In a young scallop about 1 millimeter in size (phase 5) the following 

 rate of traveling was observed. The heart beats faster when the scallop 

 is crawling than when the animal is resting. Occasionally all visible 

 cardiac movement ceases for short periods during the resting stage. The 

 rate of beat when crawling is about 100, when resting 85, per minute. 

 The first series of continuous movements permitted the animal to cover 

 the space of 5 millimeters in thirty seconds. On a second trial the 

 scallop was able to cover the same distance in twenty seconds, taking six 

 movements. Taking an approximate average the scallop would be able 

 to cover 1 inch in two minutes, or thirty times its length, if it traveled 

 consecutively. 



The crawling stage can be divided into three periods: (1) swimming 



