148 ANNULOSA. 



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 loth, nth, i2th, i3th, and i4th 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 ap- 

 pendages, which are used as swimming organs, and which are 

 termed the " swimmerets." Each swimmeret (fig. 94, 2) con- 

 sists 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 forma- 

 tions except the very oldest ; as might have been expected 

 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 individuals which actually 

 underwent fossilisation. The Crustaceans appear to have com- 

 menced 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 exclusively 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 been a marked progression in proceeding 

 from the older formations to the present day. The Trilobites 

 and Eurypterids of the older Palaeozoic Rocks, though highly 

 organised so far as their type is concerned, are in many respects 

 inferior to later forms, whilst they present some striking points 

 of resemblance to the larval forms of the higher groups. The 

 great group of the Stalk-eyed Crustaceans undoubtedly the 

 highest of the entire class is not represented at all till we 

 reach the Carboniferous Rocks : and it is not till we come into 

 the Secondary period that we find any great development of 

 this group, whilst its abundance increases to a marked extent 

 in the Tertiary period, and it attains its maximum at the 



