_Acorn-tongued worm_: Notice the very simple form and structure of the symmetrical body, the "proboscis," the collar surrounding the neck with its simple rod of cartilage, the marks of internal gills and gill slits extending some distance along the body, and the presence or absence of sense organs. The acorn-tongued worm (_Balanoglossus_) lives in the sand of the seashore and in shallow water in temperate and tropical regions. _Lancelet_: Observe the form of the body, of the fin, and of the mouth; note the presence or absence of sense organs, and find out the number of gills or gill slits. The lancelet (_Amphioxus_) is similar in habit to the acorn-tongued worm. By day it lies buried with only the mouth exposed, but at night it swims actively about. It is somewhat more confined to the tropics. _Lamprey_: Observe here also the primitive or unspecialized form of the body, of the fin, of the jawless mouth, the number of gill slits, and the sense organs. _Shark_: Examine the body, noting its form and differentiation into regions, its covering, its fins, mouth, gill slits, and sense organs. _Perch_: If you have not already studied the bony fish, the points suggested for the shark will be sufficient for this exercise. In each case, find out the condition of the skeleton. _Questions._ 1. Which of these animals seem most simple in form, and which most complex? Give a reason for your answer. 2. Give the stages which show how the fold of skin develops into separate fins. 3. How does the number of gills and gill slits change in the series? (Give definite numbers.) How may the reduction in the number of gills be compensated for in the amount of surface exposed for the exchange of gases in breathing? 4. How is protection afforded the delicate structure of the gills in the final form? 5. Give the stages in the formation of a definite, symmetrical mouth with jaws of equal size. 6. The presence of sense organs may be taken to indicate that there is an organ of control, or brain. How is the development of this organ like or unlike that of the other structures in the series? 7. For the developing brain and nervous system what protection and support is afforded in each case? The foregoing questions may be answered in tabular form by arranging the names of the animals in a line and the questions in a column. _Suggested drawings._ a. Acorn-tongued worm, × 1. b. Lancelet, × 1. c. Lamprey, × ½. d. Shark: 1, head as far as the pectoral fins; 2, the tail. e. Perch, as directed for shark. #B. STUDIES OF AMPHIBIA# #Progress from Water-living Animals to Land-living Animals# The Living Frog or Toad _To show how an Animal may be adapted to both Land and Water Life_ _Materials._ Living frogs in small cages or aquaria; living toads; some pungent liquid, as ammonia. _Observations._ _The Body._ 1. Contrast the body of the frog with that of the fish as to regions, shape, and compactness. How do you account for the differences?