n6 MORPHOLOGY OF INVERTEBRATE TYPES 



cuticle stretches rapidly, while soft, and then hardens to take 

 the place of the old skeleton. Another important feature of the 

 external skeleton is that locomotion would be impossible if 

 this were not subdivided into articulating segments connected 

 with each other by thin and pliable cuticle. This is exactly 

 what occurs in all arthropods. In higher forms the articulation 

 is so perfect that locomotion is possible only in strictly circum- 

 scribed planes. But in Daphnia articulation is still very primi- 

 tive, special articulating surfaces are still absent and the joints 

 have more freedom. The cuticle appears often sculptured. Thus 

 for example the shell of Daphnia presents the appearance of 

 a network, due to minute ridges of the cuticle. Two other 

 characteristics of an external skeleton may be mentioned in 

 this place. One is that the muscles are of necessity inside the 

 skeleton, not outside as in the case of an internal skeleton such 

 as in vertebrates. The other is that the skeleton has to be pro- 

 vided with pores to admit of a connection between the nervous 

 system and the sense organs which convey to the former the 

 impressions of the outside world. We shall see later how some 

 of the sense organs are structured. 



Appendages. The first pair of appendages are the 

 antennules (often called first pair of antennae), which belong to 

 the second cephalic somite. They are single-jointed, tiny, cy- 

 lindrical organs situated near the forward angle of the head. 

 At their ends are several olfactory bristles (aesthetascs). The 

 antennules of the male are considerably longer than those of the 

 female. The second pair of appendages are the antenna (called 

 also second pair of antennae). They are built on a strictly 

 two-branched or biramous plan and consist of several joints. 

 The joint by which they are attached to the head is called the 

 coxopodite, the one following it, the basopodite. Together these 

 two joints represent the so-called protopodite of a typical bira- 

 mous appendage. The two branches of the antenna attached 

 to the basopodite are known as endopodite and exopodite. The 

 former is three-jointed, the latter four-jointed. The endopodite 



