xi PHYLUM ARTHROPODA 551 



antennules and antennae are usually well developed, and the latter 

 are sometimes biramous : they may both be used as swimming 

 organs or for prehension. The mandibles may be provided with a 

 palp. The young is a nauplius. In the parasitic forms more or 

 fewer of these general characteristics may become lost in the 

 adult. 



ORDER 1. EUCOPEPODA. 



Free or parasitic Copepoda without paired compound eyes. 

 The appendages of the body-segments are devoid of a flagellum. 

 The genital apertures are situated on the seventh body-segment. 

 In this group are included (a) free-swimming forms, such as 

 Cyclops (Water-flea) (Fig. 458), and (6) parasitic forms or Fish- 

 lice e.g., Ergasilus, Chondr acanthus, Lerncea (Fig. 459). 



ORDER 2. BRANCHIURA. 



Parasitic Copepoda with compound eyes and a suctorial mouth. 

 Some of the appendages of the body-segments are usually provided 

 with peculiar appendages the flag ] ella. The genital apertures are 

 situated on the fifth body-segment. This order includes the 

 Carp-lice, Argulus (Fig. 460), and two other genera. 



Sub-class IV. Cirripedia. 



Imperfectly segmented Crustacea, which are always fixed in 

 the adult condition, and may be parasitic. There are usually six 

 pairs of biramous cirriform appendages of the body region. The 

 limbless posterior region (abdomen) is rudimentary, and is usually 

 provided with a pair of caudal styles. The carapace forms a pair 

 of folds, the mantle, completely enclosing the animal, and usually 

 supported by a system of calcareous plates giving rise to a hard 

 shell. Paired eyes are absent in the adult. The antennules of 

 the larva give rise to organs of attachment and become vestigial 

 in the adult : the antennas usually disappear. The mandibles have 

 no palp. The sexes are united in the great majority. The 

 young animal is hatched in the nauplius form and passes later 

 through a stage the cypris stage in which it is provided with 

 a bivalved shell. 



ORDER 1. EUCIRRIPEDIA. 



Cirripedia some of which are parasitic, while the rest are non- 

 parasitic but are permanently fixed in the adult condition. 

 There are usually six pairs of biramous trunk appendages. 



In this order are included (a) fixed forms such as Lepas 

 (Barnacle) (Fig. 461) and Balanus (Acorn-shell) (Fig. 462) and 

 (6) parasites e.g., Petrarca, Alcippe, Proteolepas. 



N N 2 



