XI PHYLUM ARTHROPODA 565 



Ohder 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 flagdla. The genital apertures are 

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

 Carp-lice, Argulun (Fig. 453) 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 antenniiles 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 arc included (ff) fixed forms such as Lepas 

 (Barnacle) (Fig. 454) and Bahinus (Acorn-shell) (Fig. 455) and (&) 

 parasites — e.g., Petrarca, Alcippe, ProUolcpas. 



Order 2. — Rhizocephala. 



Parasitic Cirripedia in which the body has undergone extreme 

 degeneration, and has lost all trace of appendages and of ali- 

 mentary canal in the adult condition. 



Including Sacculina (Fig. 456) and PeUogastcr. 



Sub-class v.— Malacostraca. 



Crustacea in which the body is always distinctly segmented and 

 is made up in all cases except the Leptostraca of an anterior region 

 (thorax) of eight segments, and a posterior (abdomen) of six, with a 

 terminal tail-piece or telson — the total number of segments, 

 leaving the prostomium and the telson out of account, being always 

 nineteen. The appendages of the thorax and abdomen are sharply 



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