144 ANNULOSA. 



are more or less completely protected by an external skeleton 

 formed by the deposition of horny (chitinous) matter in the integu- 

 ment. 



The division Arthropoda includes four great classes of 

 animals which are very generally spoken of as the " Arti- 

 culate Animals." These classes are the Crustacea (Lobsters, 

 Crabs, &c.), the Arachnida (Spiders, Scorpions, &c.), the 

 Myriapoda (Centipedes and Millipedes), and the Insccta (In- 

 sects). All these classes came into existence in the Palaeo- 

 zoic period, the division being represented by Crustaceans as 

 early as the Upper Cambrian at any rate, and doubtfully in 

 the Lower Cambrian. Owing to the fact that the Crustaceans 

 alone lead an habitually aquatic life, the remains of this class, 

 as might be expected, preponderate largely over those of the 

 other three. The air-breathing classes of the Arachnida, 

 Myriapoda, and Insecta, naturally, have not left abundant 

 traces of their existence in past time, a state of things which is 

 assisted by the nature of their integuments, which are rarely 

 as hard and resisting as those of the Crustaceans. 



CLASS I. CRUSTACEA. 



The Crustaceans are Articulate animals in which the breath- 

 ing organs (when distinct) are in the form of gills, and the mode 

 of existence is almost always more or less aquatic. The body is 

 protected by a chitinous or sub-calcareous exoskeleton or "crust" 

 and the number of pairs of articulated limbs is generally from five 

 to seiwi. Some of the locomotive appendages are often carried 

 upon the segments of the abdomen, and there are two pairs of 

 jointed feelers or " antenna" 



The body of a typical Crustacean, such as a Lobster (fig. 

 94), consists of a definite number of somites placed one be- 

 hind the other, and divisible into three regions a head, thorax, 

 and abdomen. Most authorities regard the body as being 

 typically composed of twenty-one somites, of which seven go 

 to the head, seven to the thorax, and seven to the abdomen. 

 All these somites, except the last, may be provided with a pair 

 of appendages each. The last segment of the abdomen, how- 

 ever, never carries any appendages. This segment is known 

 as the "telson" (fig. 94, i, /), and it is variously regarded as 

 a somite without appendages, or as an impaired appendage 

 placed in the middle line of the body. If this latter view be 

 adopted, the body of a typical Crustacean will consist of only 

 twenty segments, instead of twenty-one. The telson is very 



