262 . ELEMENTARY BIOLOGY. [CHAP. 



vessels ; for the study of these recourse must be had 

 to transverse sections. (Cf. Sect. G. y.) 



d. The blood fluid. Lay open, high and dry, the 

 body cavity of a freshly-killed worm, and introduce 

 a fine capillary tube into one of the larger vessels. 

 Transfer the drop of red blood thus obtained to a 

 slide, cover and examine at once under a high 

 power. Note — 



u.. The blood serum, a watery yellowish-red fluid in 

 which float 



/?. minute colourless corpuscles. In shape, they are 

 somewhat irregular (rarely oval); each usually 

 contains a central dark granule. 



If unsuccessful by the above method, remove 

 a small portion of one of the larger vessels en 

 masse between two pairs of forceps ; transfer to 

 a slide and allow the fluid contents to escape. 



G. The nervous system. 



Dissect an entire worm from above, removing the dorsal 

 portion of the body wall, and the alimentary canal from the 

 pharynx backwards. Pin down, wash until quite clean, and 

 dissect under water. Examine under a lens and note — 



a. The supra-aesophageal ganglia; two pear-shaped 

 masses lying in the third segment in a depression 

 between the buccal-sac and pharynx. (Cf. Sect. 

 D. i. b.) Fibres can be traced from them to the 

 circum-oral integument. 



b. The circumxsophageal commissures ; seen, on dis- 

 placing the pharynx to one side, to arise from a. 

 The nerves arising from them are distributed — 

 externally, to the first two segments — internally, to 



