ORGANIC EVOLUTION 



179 



wall the distribution of muscle bands and the location of the 

 rows of setae. 



The record of this study may consist in a carefully pre- 

 pared diagram to show the relation of the organs (except 

 reproductive and excretory) in the median plane of the 

 body. Also, a tabular statement of the parts that are 

 repeated in each segment: a) organs; b) parts of organs. 



Study 25. The cellular structure of the earthworm. 



Materials needed: Live worms and prepared slides of 

 cross sections. 



Examine the blood of the worm (taken with a pipette 

 from the ccelom of an anesthetized specimen) for leucocytes, 

 bacteria, loose chloragogue granules, etc. 



Examine a drop of the fluid from the sperm vesicles for 

 sperm cells in clusters in various stages of development. 



Examine a mounted ovary, either fresh or a stained 

 preparation. 



Study cross sections of the worm's body and identify 

 all the tissue composing it. 



The record for this work may consist in drawings of leuco- 

 cytes, sperm cells, egg cells in the ovary, and a few typical 

 cells from the more important tissues of the body, such as 

 epithelium and hypodermis. Also a tabular statement 

 of the spatial relations of the tissues passing from the 

 outside inward in: a) body wall, and b) enteron wall. 



THE SALAMANDER. 



The spotted salamander (Ambystoma tigrinum) is a very 

 common vertebrate, of rather primitive structure. It will 

 serve very well to illustrate that type of animal organization 

 that is found in our own bodies. It is nocturnal and very 

 secretive in its habits, being often seen in cellars and base- 

 ments, although occasionally dug up in moist garden soil, 



