THE COPEPODA 



75 



lated wiih 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 with the eye. 



The two rami of the caudal furca are usually movably 

 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 become converted into hook-like organs of 

 adhesion (Doropygus). Between the furcal rami and somewhat on 

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



FlO. 42. 

 Calocalanus pavo, x 10. (After Giesbrecbt) 



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 be mentioned 

 here is the development in several Ascidicolidae of a dorsal brood- 

 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 body. 

 The free edges of the thoracic somites may be produced into 

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

 dorsal surface (Notopterophorus). 



Appendages. The antennules are always uniramous, and in many 



