532 THE ANIMAL KINGDOM. 



the fused somites of the thorax form the branchiostegite of Podophthal- 

 mata and the bivalve shell of the Estheridae (Branchiopoda], Cladocera, 

 and Ostracoda. In the last-named group and in Nebalia a special trans- 

 verse muscle closes the shell upon the body. 



The first antenna is primitively uniramose, and retains this character 

 in Phyllopoda, Ostracoda, and Copepoda ; but during growth it generally 

 becomes two or three branched. It is minute in Cladocera and terrestrial 

 Isopcda. In the Cirripedia its second joint carries a disc in connection 

 with which is the aperture of a cement gland. The animal is attached by its 

 means to some foreign object, whilst the head dilates into a broad base 

 or elongates into a peduncle. The other appendages of the head and body 

 are primitively biramose, except in the Ostracoda and the appendages of 

 the thorax in Arthrostraca. The second antenna may become uniramose, 

 or the outer branch may be reduced to a scale or squame (many Thora- 

 costracd}. It is minute in Apus and is lost in all Cirripedia and Hyperidae 

 (Amphipoda). The mandible is reduced to the basal masticatory portion in 

 Phyllopoda and Cumacea. In other groups the reduction may be temporary, 

 and there are developed subsequently a number of small joints which form 

 \hepalp. In many parasitic suctorial Crustacea the mandibles form a pair 

 of stylets, inclosed in a more or less complete sheath constituted by the 

 so-called upper and lower lips. The maxillae vary much in character, but 

 are generally more or less expanded or foliaceous. The outer and inner 

 branch ( = exo- and endo-podite) of the second maxilla are separated in 

 Copepoda so as to appear like independent limbs, whilst in Cladocera the 

 appendage is aborted. In the Copepodan families Calanidae and Pontellidae 

 both first and second maxillae closely resemble the biramose Na^^plius 

 appendage. The first maxillae, like the mandibles, are stylets in Argulus, 

 whilst the second are converted into large sucking discs. Both pairs of 

 maxillae want the exopodite in Isopoda. 



The thorax is always limb-bearing ; the abdomen bears none in 

 Entomostraca, but does so with few exceptions in Malcostraca. The 

 primitive type of limb is probably that of the Copepoda, which closely 

 resembles the Nauplius appendage. It has a basal stem carrying a more 

 or less jointed or lamellate exo- and endo-podite. Such a limb is seen in 

 the thoracic appendages of Cirripedia and of the Schizopoda among Mala- 

 costraca, and is generally found in the abdominal region. The Phyllopod 

 type of appendage, seen also but in a more primitive state in Nebalia, with 

 its branchia and external respiratory plate, and its series of internal lobes 

 or endites, is probably an adaptation of the Copepodan limb. The 

 exopodite is lost in the ambulatory thoracic limbs of Arthrostraca and 

 Decapoda. Of the eight pairs of thoracic limbs in Malacostraca the first 

 pair in Arthrostraca, the three first in Decapoda, and the five first in 

 Stomatopoda are modified into foot jaws or maxillipeds. Apus (Branchiopoda} 



