332 ARTHROPOD A. 



The eleventh segment (the fourth of the thorax) carries a 

 second pair of limbs, also " chelate," but much smaller than 

 the preceding ; and the twelfth segment (the fifth of the 

 thorax) carries another pair of the same. The thirteenth 

 segment (the sixth of the thorax) carries a pair of limbs like 

 the preceding, but with simply-pointed extremities ; and the 

 fourteenth segment (the last of the thorax) carries another 

 pair of the same ; so that there are altogether five pairs of 

 ambulatory limbs, carried respectively by the 10th, llth, 

 12th, 13th, and 14th somites of the body; or, in other 

 words, by the last five segments of the thorax. Of the seven 

 segments of the abdomen completing the total of twenty- 

 one the first six carry each a pair of appendages, which 

 are used as swimming organs, and which are termed the 

 " swimmerets." Each swimmeret (fig. 194, 2) consists of a 

 propodite and a flattened exopodite and endopodite ; and the 

 last pair is greatly widened out and expanded, forming with 

 the telson a powerful swimming -tail. The telson or last 

 abdominal segment carries no appendages, and is simply 

 placed between the last pair of swimmerets. 



As regards the general distribution of the Crustacea in 

 time, remains of the class are comparatively abundant in all 

 formations except the very oldest ; as might have been ex- 

 pected from the generally chitinous or sub-calcareous nature 

 of their integuments and their aquatic habits. Owing also 

 to their habit of periodically casting their shell, a single 

 individual may leave repeated traces of himself, and the 

 number of fossils may considerably exceed that of the indi- 

 viduals which actually underwent fossilisation. The Crus- 

 taceans appear to have commenced their existence in the 

 Cambrian period, remains of members of this class being 

 tolerably abundant in the higher portion of this formation. 

 The Palaeozoic formations, taken as a whole, are characterised 

 by the predominance of the orders Trilobita, Eurypterida, 

 Ostracoda, and Phyllopoda, of which the two former are ex- 

 clusively confined to this period. All the other orders of 

 Crustacea, which have left any traces of their past existence 

 at all, appear to have come into existence before the close of 

 the Palaeozoic period. Upon the whole, however, there has 



