DEVELOPMENT 491 



various degrees ; at the thickest spot about 50 individuals in a 

 cubic foot of water, while at the thinnest only about 10 in the 

 same. After nearly 100 yards row, we came across another 

 broader sheet of swimming Balanoglossus. There they were 

 so thick that we could count nearly 100 specimens in a cubic 

 foot of water. When the sun was up, this curious plankton 

 almost suddenly disappeared." 



This circumstantial account is interesting in showing that 

 what we are accustomed to think of as a somewhat rare type 

 may be very abundant in certain places and at certain times. 

 Ikeda's picture is a fine instance of the prodigal abundance of 

 life, and he adds another graphic touch : " On coming back 

 to shore, we found, to our great surprise, a considerable stretch 

 of the beach (one metre in width) covered with the deep reddish- 

 brown Enteropneusts ". 



The species of Balanoglossus which was here observed was 

 Glandiceps Ziacksii, and there are some instructive points to be 

 noticed. (1) The ventral side is much lighter than the dorsal, 

 which we may associate with the animal's habit of creeping on 

 the floor of the sea at a depth of 5 to 15 fathoms. (2) The 

 posterior region of the body is much flattened and the margins 

 function as fins in the swimming. (3) The food-canal contained 

 no sand, but compacted masses of diatoms and infusorians 

 (dinoflagellates). It seems then that this Enteropneust is not 

 a burrower or sand-eater, like most members of the class, but 

 a creeper and swimmer. As " the swarming has nothing to do 

 with sexual maturity " it may be concluded that " this form 

 comes to the surface after microplankton, which, as we know, 

 flourishes especially in summer months, and is most abundant 

 in calm mornings before sunrise." 



The most remarkable general fact regarding the develop- 

 ment of Enteropneusts is that two distinct modes occur. 

 This is striking when we consider that the class is relatively 

 a very small one. In the family Balanoglossidae what is 

 called direct development obtains, that is to say, the egg 

 develops into an embryo which quite gradually develops into 

 a miniature of the adult worm. In the families Ptycho- 

 deridae and Spengelidae, what is called indirect development 

 obtains, that is to say, the egg develops into a larva which is 

 quite unlike a miniature of the adult, and only passes on to 



