XII] ASTACUS 1*79 



When the abdomen is bent upwards each tergum is found to 

 articulate with both its successor and predecessor by two points 

 situated one at each side. Each tergum slides to a certain 

 extent under its predecessor: and each has a smooth part called 

 the tergal facet, over which its successor slides. 



The last division of the abdomen is a flat semicircular piece 

 without appendages, which is divided into two by a transverse joint 

 and on the under surface of which the anus is situated. This is 

 called the t els on, and in the lower Crustacea it is represented by 

 a pair of appendages called the caudal fork between which the 

 anus lies it is believed that in the crayfish these appendages 

 have coalesced to form the telson. The last regular segment 

 bears a pair of appendages, both forks of which exopodite and 

 endopodite have taken on the form of broad triangular valves 

 (xx, Fig. 71). These can be folded the one under the other and 

 the whole limb can be concealed under the telson but they can also 

 be spread out at the sides of the animal's body so as to make with 

 the telson a broad terminal fan. 



By means of this fan the animal is able to execute a manoeuvre 

 which is of the utmost value to it when it is pursued by its enemies. 

 It bends the abdomen quickly and sharply downwards, striking the 

 water with the outstretched fan and the reaction of the water drives 

 the whole animal suddenly backwards. In this way the crayfish 

 can back into a crevice between stones, whilst keeping its eyes fixed 

 on its enemy. 



In the case of the next three segments of the abdomen the 

 protopodite is composed of a stout cylindrical sclerite. Both exo- 

 podite and endopodite on the contrary are made up of a large number 

 of short disc-like joints, so that these divisions of the limb are 

 exceedingly flexible. They are furthermore fringed with "hairs," 

 that is to say delicate spines of cuticle in no way comparable to the 

 structures -called hairs in Mammalia (see p. 636). It is instructive 

 to notice the contrast between these structures and the chaetae of 

 Chaetopoda. The Chaetae as we have seen are solid pillars of cuticle 

 in which the oldest part is the tip. The "hairs" are on the contrary 

 hollow structures containing at first an axis of ectoderm. The term 

 seta has been proposed for them, and it is preferable to hair as the 

 latter term suggests a false comparison. 



The appendages under discussion those belonging to the third, 

 fourth and fifth segments of the abdomen are termed swimmerets 

 (sw, Fig. 70) and are much better developed in some of the lower 



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