ELEMENTARY BIOLOGY. 



ft. Turn the animal over and note- the very 

 narrow sterna between the points of attach- 

 ment of the thoracic appendages. 



,'.,- : The last thoracic somite is not. ankylosed 

 ...n-, with the rest in the crayfish. In the lobster 

 . it is. 



7. Eaise with a pair of forceps the free edge of 

 the lateral part of t'he carapace which lies just 

 over the bases of the thoracic appendages, and 

 is termed the branchiostegite : note that it is 

 formed by the large united pleura of the 

 thoracic segments, and overlaps a chamber in 

 which the gills lie. 



9. Note the plane in which the sterna of the anterior 

 three somites pf the animal (marked out by their 



r* appendages) lie it is nearly at right angles to the 

 plane of the remaining sterna of the cephalo thorax 

 so that their appendages are directed forwards in- 

 stead of downwards. 



10. Cut a vertical section of a piece of the exoskeleton 

 . which has been decalcified by lying in Ig- chromic 



acid solution for a few days. 



a. It will be seen to be composed of a large number 

 of parallel laminae which are thicker towards 

 the outer part. The laminae are marked by ill- 

 defined parallel lines which run perpendicular to 

 the surface, and which give their edges a striated 

 appearance. The outermost layer is more trans- 

 parent than the rest and wants this striation. 



b. The epidermis lying beneath the innermost of the 

 above laminae is composed of ill-defined branched 

 nucleated granular cells: the outermost giving off 

 a large number, of short processes, which end in 



