THE LOWER VERTEBRATES 157 



border-line between Invertebrates and Vertebrates. On some 

 shores, e.g. certain parts of the Clyde estuary, there is great 

 wealth of what are called compound Ascidians underneath stones 

 about low- tide mark. These are even more difficult to under- 

 stand than ordinary sea-squirts, but they should not be missed, 

 since they often have a quite remarkable jewel-like beauty. 



Summary. Below the level of fishes there are at least three 

 classes of more primitive Vertebrates each peculiar on its own 

 line the Cyclostomes, the Lancelets, and the Tunicates. The 

 existence of these classes suggests that the vertebrate series 

 like the other great series in the animal kingdom began gradually 

 in this sense, at least, that it was originally represented by forms 

 somewhat like " niners " or Palceospondylus, or like larval lance- 

 lets or like larval tunicates, only very much simpler and without 

 the special peculiarities which mark all these types. A reference 

 to any good text-book on zoology will show that there are some 

 remarkable more or less worm-like types, e.g. Balanoglossus, 

 which serve still further to lessen the apartness of the Vertebrates. 



Retrospect. We have completed a survey of the backboned 

 animals from a certain point of view, and we wish to conclude 

 with two general suggestions. While classification or orderly 

 grouping is only a very small part of nature study, it is a valuable 

 part. Practice in classification is a good discipline ; it is often 

 an exercise in judgment as well as in precision, and it tends 

 towards clear thinking. Why is a whale not a fish ? why is 

 the sea-horse a fish ? why is a slow-worm a lizard, not a snake ? 

 why is a newt an amphibian, not a reptile ? and so on these 

 are not useless puzzles of our own making, they are common- 

 sense questions quite useful in their way. Again, it is desirable 

 as part of the A B C of education that there should be in the 

 mind a general picture of the animal kingdom showing the re- 

 lative position of the chief classes. This is often missed because 

 the wood is lost in the trees. Yet it should be an elementary 

 exercise to arrange on a table in an orderly way a score or so 

 of handy specimens of common animals. It should be possible 

 to have at least one cupboard with a glass door, whose shelves 

 show at a glance the general " lie " of the great types of animals. 

 With a little care and pertinacity it will be possible to secure 



