On the Swimming Habit of a Japanese Enteropneust, 

 G/andiceps hacksii Marion. 



BY 



Iwaji Ikeda, Rigakushi. 



Very early in the morning of September 3, 1907, when I was out 

 skimming with some of my students a short distance off Sesuijima (near 

 Tomo about 50 miles E. of Hiroshima) in the Inland Sea, a curious sort of 

 plankton covering a considerable area attracted our notice. On examining 

 the contents of our net, it turned out to be swarming Balanoglossus. A 

 little later, when the sun was about to rise, we could perceive myriads of 

 lively swimming specimens about our boat. We now came to realize that 

 we had been" rowing about in a big sheet of swarming Enteropneusts. 

 More than delighted with this sight, we collected a bucketful of specimens 

 — a task accomplished in but a minute. They measured from three to 

 fifteen cm. in length (eight cm. on an average). The belt-like zones of 

 this plankton varied from one to five metres in width and were in some 

 cases two metres in thickness. The animals were crowded in various 

 degrees ; at the thickest spot about fifty individuals in a cubic foot of 

 water, while at the thinnest only about ten in same. After nearly a hund- 

 red yards' row, we came across another broader sheet of swimming Balano- 

 glossus. There they were so thick that we could count nearly a hundred 

 specimens in a cubic foot of water. When the sun was up, this curious 

 plankton almost suddenly disappeared. On coming back to the shore, we 

 found to our great surprise a considerable stretch of the beach (one metre in 

 width) covered with the deep reddish brown enteropneusts. Most of the 

 stranded specimens were mutilated, the post-hepatic region being lost. 

 They were naturally very sluggish out of their element ; however, they 

 still showed some movements to burrow into the sands with their proboscis. 



