144 ELEMENTARY BIOLOGY. [CHAP. 



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 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 

 clubbed ends and penetrate a short way into the 

 exoskeleton. 



11. The respiratory organs. Remove now the branchio- 

 stegite on one side and examine the gills: they are 

 1 8 in number, arranged in two sets. 



a. Six are attached to the epipodites of some of 

 the appendages (2nd and 3rd maxillipedes, 

 chelae, ist, 2nd, and 3rd pair of ambulatory 

 limbs). 



P. The remaining 12 are fixed to the sides of 

 the body, and each consists of a central stem 

 giving off a number of delicate filaments. 



