138 ELEMENTARY BIOLOGY. [xil. 



yS. Turn the animal over and note the very 

 narrow sterna between the points of attach- 

 ment of the thoi'acic appendages. 



The last thoracic somite is not ankylosed 

 with the rest in the craj'fish. In the lobster 

 it is. 



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

 the lateral part of the 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 of the animal (marked out by their 

 appendages) lie — it is nearly at right angles to the 

 plane of the remaining sterna of the cephalothorax — 

 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 1§ 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 laminse is composed of ill-defined branched 

 nucleated granular cells : the outermost giving off 

 a large number of short processes which end in 



