202 ELEMENTARY BIOLOGY. [CHAP. 



c. The pro-cephalic process; a tongue-shaped ingrowth 

 of the exoskeleton, at the base of the rostrum. 



d. The thoraco-abdominal link work; a somewhat com- 

 plicated system of bars, passing between the last 

 two thoracic sterna and a special process of that of 

 the first abdominal segment. 



e. The aperture of the mouth; bounded by the man- 

 dible note the natural relations of the latter. The 

 upper-lip or labrum and the lower-lip or metastoma 

 will be seen, as uncalcified prolongations of the 

 sternal skeleton, situated, respectively, in front of 

 and behind the mouth. Compare the same as 

 looked at from beneath, in an uninjured specimen 



a. The labrum ; a shield-shaped plate calcified mar- 

 ginally. 



/?. The metastoma; composed of a median fold, pro- 

 duced on either side into a blade-like process, 

 which simulates the basal joint of the first maxilla. 



/ Note the limits of the several segments of the axial 

 skeleton, and the mode of articulation of each upon 

 its fellow and of the appendages upon each. 



6. The histology of the exoskeleton. 



a. Make a thin longitudinal-vertical section of any of 

 the inter-articular membranes, and examine in water 

 under a high power. It consists of a highly elastic 

 membrane, traversed longitudinally by regularly 

 disposed clear striae; if pressure be applied to the 

 cover-slip, the layers obvious as the above-named 

 striae will be found to be denser and more resistant 

 than the intervening ones, i.e. the whole is composed 



