CRUSTACEA. 21 



that this part is really a ring whose appendages are 

 wanting in the lobster. 



If the pupils have spent sufficient time upon this 

 portion of the lobster's body, they cannot fail to see 

 that it is made up of rings, and that to every ring there 

 is normally a pair of appendages. Also, that all the 

 appendages are built upon the same fundamental type, 

 though differing in the details of form. 



The reason for this difference becomes evident when 

 the habits of the living lobster are observed. The ap- 

 pendages of the sixth ring are supported by the telson, 

 and, together with it, form a tail-fin, which is the chief 

 organ of locomotion. This organ, aided by the flexible 

 abdomen, strikes down against the water like a paddle, 

 and with such force that the reaction sends the animal 

 swiftly backwards. By doing work which requires so 

 much effort, the last pair of appendages, as might be 

 expected, are large and powerful. The fifth, fourth, 

 third, and second pairs (Fig. 8, C, f 2-f 5) are also 

 pendant paddles, though less efficient, and are, there- 

 fore, similar in aspect, but not as large. They enable 

 small lobsters to swim in a forward direction and assist 

 the older ones in sustaining themselves when floated 

 up from the bottom, where the adults habitually crawl. 

 They are constantly in motion, accomplishing little, 

 however, as locomotive organs in the full-grown ani- 

 mal, but in the female they are used for carrying the 

 eggs during the period of oviferation. 



The sexual difference between male and female is 

 shown by the condition of the first pair of swimmerets 

 in the female (Fig. 8, C, / 1), which, being useless, 

 are rudimentary, and by the corresponding pair which 



