DRIFTING LIFE 



119 



modifications of the " foot," that solid uninteresting mass 

 on which most shellfish creep (Fig. 23). These animals 

 are also so numerous in some parts of the ocean that their 

 empty shells form deposits on the sea floor, the Pteropod 

 Ooze (see page 56). 



It is a characteristic of nearly all the plankton animals 

 that live in the upper layers of the sea, that they are almost 

 transparent. If one looks at a tow-net catch that has been 



"iMrjp 



Fig. 24. 

 Sagitta (X2). 



Fig. 25. 

 Tomopteris (x 2) 



placed in a glass jar full of sea water, it is at first very hard to 

 see the various animals on account of their transparency. A 

 very common creature in the catch is the " arrow worm," 

 or Sagitta (Fig. 24) ; this is thought to be a relative of 

 the true worms, like the rag- worm, although it is very 

 unlike them in appearance. It looks just like a little glass 

 rod about three-quarters of an inch in length ; but for all 

 its apparent delicacy it is a very voracious creature. Sur- 



