xi PHYLUM ARTHROPODA 563 



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 

 (Genypterus blacodes), and the rest of the body hanging freely 

 into the water. Lastly, in Tracheliastes (6) the second maxillae 

 (mx.2) are greatly enlarged, and form a characteristic organ of 

 attachment. 



Argulus (Fig. 460) is the most familiar example of the Branchiura, or 

 Carp-lice. It is an external parasite on fresh- water Fishes (Carp, Stickleback, 

 &c.), not permanently attached like the degenerate forms just described, but 

 crawling freely over the surface of the host. The body consists of an oval 

 flattened cephalothorax, and a small bilobed abdomen (db. ). The mandibles 

 and maxillae are piercing organs enclosed in a sucking-tube or proboscis (r.), 

 in front of which is a median tube ending in a spine (st.). The second maxillae 

 are divided into two portions, the anterior of which (kf. 1} are modified into 

 sucking-discs by which the parasite clings to the surface of its host, and there 

 are four pairs of swimming-feet (b? 6 4 ). Alone among the Copepoda the 

 Branchiura have no egg-sacs, and they are exceptional also in the possession 

 of compound eyes (pa-). 



The most familiar examples of the Eucirripedia are the Barnacles 

 found on ships' bottoms, piles, &c., and the Acorn-shells or Sessile 

 Barnacles which occur in immense numbers on rocks between 

 tide-marks in all parts of the world. 



The common Barnacle (Lepas anatifera) is attached by a long 

 stalk or peduncle (Fig. 461, A, p), covered with a wrinkled skin, 

 and bearing at its distal end the body proper enclosed in a sort of 

 bivalved carapace, formed by a fold of the skin, and strengthened 

 by five calcareous plates. Of these one is median and dorsal, and 

 is called the carina (c) ; two are lateral and proximal, the scuta (s) ; 

 and two lateral and distal, the terga (t). During life the carapace 

 is partly open, and from the ventrally placed aperture delicate 

 setose filaments are protruded and keep up a constant grasping 

 movement : these are the endo- and exopodites of the biramous 

 thoracic feet, of which there are six pairs. Removal of the carapace 

 shows the feet to arise from a vermiform unsegmented body (B), 

 attached on the ventral aspect to the stalk and carapace by its 

 anterior end, while its posterior end is free and terminates in a 

 long filament, the penis (p), immediately dorsal to which is the 

 anus. The mouth is ventral and anterior, and is provided with 

 a pair of mandibles and two pairs of maxillae. There are no 

 antennae ; at first sight the antennules appear to be absent, but 

 a careful examination shows the presence of a pair of minute 

 structures (of) on the proximal or attached surface of the stalk, 

 and embedded in the cement by which the animal is fixed to its 



