TYPE OBUSTAOEA. 393 



3. Order Copepoda. 



The members of the order Copepoda present great varia- 

 tions in form, due to the fact that there are a number of para- 

 sitic forms belonging to it some of which show so much de- 

 generation that their relationships to the non-parasitic forms 

 only become apparent by a study of their development. 

 Typically, however, the body is generally elongated (Fig. 177) 

 and consists of ten segments in addition to those of the head, 

 the five anterior ones usually bearing appendages and con- 

 stituting the thorax, while the five posterior lack appendages 

 and form the abdomen, the terminal segment of which bears 

 a pair of caudal furcse provided with setse. In female indi- 

 viduals the two anterior abdominal segments fuse together to 

 form a genital double segment, and in all cases the head seg- 

 ments fuse together, while the anterior thoracic segment 

 usually fuses with this consolidated mass. No shell-duplica- 

 ture occurs. In the parasitic forms there is a tendency for 

 the various segments to become indistinct and all trace of 

 them may vanish, the abdomen in some cases becoming also 

 extremely reduced in size. Add to this that lobes and pro- 

 cesses are frequently developed upon the body and it will be 

 understood how far these degenerate forms depart from the 

 typical arrangement. 



The antennules (Fig. 177, af) in all free-swimming Cope- 

 poda form long many-jointed swimming-organs used in an 

 oarlike manner. They consist of a certain number of stout 

 basal joints, terminated by a single long multiarticulate 

 flagellum, no trace of a biramous condition being apparent. In 

 addition to their locomotor function they also, as in other 

 forms, serve as sense-organs, olfactory hairs being scattered 

 along the flagellum, and in male individuals they are specially 

 modified to form clasping organs for use in copulation. The 

 antennae (af) are much smaller and are frequently biramous, 

 and the mandible (m«) has usually a palp, while the first 

 maxillse {mx'), bearing strong masticatoi'y bristles on their 

 basal joints, also show more or less indication of a biramous 

 condition. The second maxillse (mx'), sometimes termed the 

 maxillipeds, show a peculiar arrangement in that each 



