338 THE INVERTEBRATA / 



poda, with which they are generally placed, but differ from that class 

 in certain important features, notably in the possession of compound 

 eyes, the position of the opening of the maxillary gland on the ist 

 thoracic limb and that of the genital ducts between the 5th pair, the 

 inclusion of the maxillules in the proboscis, and the phyllopod-like 

 proximal overhang of some of the thoracic exopodites (Fig. 243 B). 



The carp-lice, as the Branchiura are called, are found both on fresh- 

 water and marine fishes. They are good swimmers. The females 

 deposit their eggs on stones and other objects. 



Argulus (Fig. 243), the principal genus, has a pair of suckers on 

 the maxillae and a poison spine in front of the proboscis. A.foliaceus 

 is common on freshwater fishes in Britain and the Continent. 



Class CIRRIPEDIA 



Fixed and for the most part hermaphrodite Crustacea; without com- 

 pound eyes in the adult ; with a carapace (except in rare instances) as 

 a mantle which encloses the trunk; with usually a mandibular palp, 

 which is never biramous ; and typically with six pairs of biramous 

 thoracic limbs. 



The great majority of the Cirripedia are extremely unlike the rest 

 of the subphylum, and would not be recognized as crustaceans at all 

 by the layman. The familiar members of the class are the ordinary 

 barnacles (Thoracica). Besides these, however, it contains several 

 groups of related organisms, of which the parasitic barnacles (Rhizo- 

 cephala) are the best known. The Asco thoracica link the class to other 

 crustaceans. 



Order THORACICA 



Cirripedia with an alimentary canal ; six pairs of biramous thoracic 

 limbs; no abdominal somites; and permanent attachment by the 

 preoral region. 



We shall take as an example of this group the common goose 

 barnacle, Lepas (Figs. 244, 246 A), found all the world over on floating 

 objects in the sea. It hangs by a stalk or peduncle which, as we shall 

 see, represents the foremost part of the head, greatly elongated but 

 still bearing at its far end the vestiges of the antennules, imbedded 

 in a cement by which it is held fast. The glands which produce the 

 cement are contained in the peduncle, and open on the antennules. 



The rest of the body is known as the capitulum, and is completely 

 enclosed in the carapace or mantle, a fleshy structure strengthened 

 by five calcified plates — a median dorsal carina, and on each side two 

 known as the scutum and tergum. The scuta are anterior to the terga, 

 that is, nearer to the peduncle. The mantle cavity opens by a long slit 



