xi PHYLUM ARTHROPODA 575 



aperture, and is fused in the female with the first abdominal 

 (abd. 1). There are four abdominal segments : the last bears the 

 dorsal anus (an), and a pair of caudal styles produced into plumed 

 setae. 



The antennules (ant 1) are very large, and are the principal 

 organs of locomotion. In the male they are modified (C.) by a 

 peculiar form of joint and long setae as clasping organs, used 

 for holding the female during copulation. The antennae (ant. 2) 

 are comparatively short and uniramous. Mandibles and maxilla? 

 are present, and the first four thoracic segments bear biramous 

 swimming-feet (la s.f.\ those of the right and left sides being 

 connected by transverse plates or couplers. The fifth thoracic 

 segment bears a pair of vestigial limbs : the abdominal segments 

 are limbless. 



Some of the pelagic marine Eucopepoda (Fig. 451, 2) are re- 

 markable for their brilliant colours and for the extraordinary 

 development of their setae, especially those of the caudal 

 styles. 



The parasitic Eucopepoda, or Fish-lice, present a very interesting 

 series of modifications, illustrating the degeneration of structure 

 which so often accompanies parasitism, Ergasilus (Fig. 452, 1) is 

 found on the gills of the Bass (Morone labrax) ; it is readily recog- 

 nisable as a Copepod, but the appendages are greatly reduced, the 

 antennae modified into hooks for holding on to the host, and the eyes 

 absent. Anthosoma (#), found in the mouth of the Porbeagle Shark 

 (Lamna cornubica), has recognisable appendages, but the form of 

 the body is much modified by the development of curious overlap- 

 ping lobes. Nicothoe (3}, found on the gills of the Lobster, has 

 antennae and mouth-parts modified for suction : the abdomen is 

 normal, but the thorax is produced into huge lobes, which give 

 it a curiously deformed appearance. In Chondracanthus (4\ the 

 various species of which are parasites on the gills of Bony Fishes, 

 there is, at the first glance, nothing to suggest that the animal 

 is a Crustacean, except the characteristic copepod egg-sacs : the 

 body is depressed, unsegmented, and produced into crinkled lobes, 

 and it requires careful examination to discover that antennules, 

 hooked antennae (ant. 2) used for attachment mandibles, maxillae, 

 and two pairs of legs (f.l, f.2) are present. The male (b) is of 

 higher organisation than the female, but of minute size about T V 

 the length of its mate and is permanently attached to her body, 

 close to the genital aperture (a, M). In Lerncea (7) and its allies 

 the body is vermiform with a curiously lobed anterior end : the 

 maxillae are adapted for piercing the skin of the host and sucking 

 its juices, and there are minute vestiges of feet. In Lesteira (5) 

 the degradation is even more marked : the female reaches a large 

 size 70 mm. in length, excluding the egg-sacs and is found 

 with the swollen head between the skin and flesh of a fish 



