THE COl'EPODA 



75 



lilted with the head in some Harpacticidae, and resembling the 

 rostral plate of the Leptostraca. More usually the rostrum is 

 bent down under the head and is often forked. A very remark- 

 able modification of the rostrum found in some Pontellidae is 

 mentioned below in connection Avith the eye. 



The two rami of the caudal furca are usually movaljly 

 articulated with the telson or terminal segment. Their setae 

 are very constant in number and position, and afford valuable 

 systematic characters. In some pelagic forms these setae attain 

 an exaggerated development (Fig. 42), while in some commensal or 

 parasitic forms they l^ecome converted into hook-like organs of 

 adhesion {JJoropi/(ji(s). Between the furcal rami and somewhat on 

 the dorsal side is the anal aperture, covered by a small supra-anal 



Fio. 42. 



Caloccilanus pavo, x 10. (After Gii'sbicclit.) 



plate which may represent the post-anal region of the telson in the 

 Branchiopod Lepidurus and the Malacostraca. 



A modification of the thoracic region which may lie mentioned 

 here is the development in several xVscidicolidae of a dorsal l)rood- 

 pouch formed by a fold of the integument arising sometimes from 

 the fourth, sometimes from the second free thoracic somite. 



The modifications which the form of the body undergoes in 

 parasitic Eucopepoda consist in the coalescence of somites leading 

 ultimately to the disappearance of segmentation, and in the develop- 

 ment of lobes and processes from the various regions of the ])ody. 

 The free edges of the thoracic somites may be produced into 

 lamellar appendages, or wing-like lobes may lie developed on the 

 dorsal surface (N'otopterophoru.'i). 



Appendages. — The antenmdes are always uniramous, and in many 



