MARINE WORMS 



189 



structure, it is interesting to note that in their young or larval state 

 the body consists of two cavities, one of which contains the internal 

 organs, while in the other the central portion of the nervous system 

 is developed, in which respects they resemble the vertebrate or 

 back-boned animals fishes, amphibians, reptiles, birds, and mam- 

 mals. At this stage, too, the creatures possess a tail that is sup- 

 ported by a rod of gristle similar to that which gives place to the 

 backbone in the developing vertebrate. These features, though only 

 transitory, are regarded as a mark of relationship to the higher 



FIG. 125. SEA SQUIRT 



forms of animal life, and thus the tunicates have been separated 

 from the sub-kingdom Vermes by some zoologists, and given an 

 exalted place at the top of the invertebrate scale, where they form 

 a sub-kingdom of their own, and are looked upon as a link con- 

 necting the invertebrates with the vertebrates. 



Before passing on to the next sub-kingdom, we should observe 

 that the interesting Eotifers or Wheel AnHr-.ls also belong to the 

 Vermes ; but although many of these minute creatures are to be 

 found in sea water, their principal home is the stagnant water of 

 fresh-water ponds and ditches, and thus we may be excused for 

 neglecting them here. 



