ARTICULATA. 113 



Fam. 6. Oniscidm. Tliese auimals are either marine or terrestrial, the 

 first pair of antennae are ruclinientary, the second only being fully developed 

 and conspicuous. The feet are slender, and formed for walking. The 

 terrestrial genera {Porcellio^2)l. 78, fig. 22) inhabit humid places, as cellars, 

 crevices in walls, beneath stones or loose bark, and similar localities, feeding 

 upon decaying vegetable matter. The female carries the eggs in a sac 

 beneath the body. When the young first make their appeai-ance, they have 

 a segment of the body and a pair of feet fewer than the adult ; and the head 

 and antennae are proportionally larger. The young are retained for a short 

 time between the respiratory laminae. Oniscus afiinis., Say, is a common 

 American species. 



The singular marine genera, Pycnogonum and Nymplion [pl.^^^figs. 32, 

 33), with but four pairs of legs, were formerly placed in this, or the preceding 

 or succeeding order, by various authors, but they arc by many believed to 

 belong to the Arachnida. 



Okdek 7. L^MODiPODA. The animals of this order are small, and present 

 an unusual aspect. The head is small, the thorax has six segments, and the 

 posterior extremity ends in a little tubercle corresponding to the abdomen. 

 There are four antennae ; the mouth has a pair of toothed mandibles, two 

 pairs of lamellar maxillae, and one pair of foot-jaws. There are five or 

 seven pairs of feet, and in the former case the missing ones are generally 

 those of the third and fourth pairs, which are represented by lamellar scales. 

 The feet are prehensile, and the first and second pairs have a kind of hand 

 capable of holding ; the former are afiixed to the head, and the latter to the 

 thorax. Branchial vesicles are attached to the second or third thoracic 

 segments, and sometimes to the first. The order contains two families. 



Fam. 1. Caprellidcß. Cajprella {pi. 78, fig. 23) is a small and very 

 slender animal with thin feet, a thick head narrowing posteriorly, and the 

 first pair of antennae larger than the second. The species live among 

 marine plants, along which they walk in the manner of a leech. In 

 swimming, they bend the posterior extremity and straighten it suddenly. 



Fam. 2. Cyamidce. Oyamus {pi. 7^, fig. 24) is a genus found in colonies 

 parasitic upon whales. 



Order 8. Amphipoda. This order takes its name from having two kinds 

 of feet, cheliform and simple, and because the feet vary in the position of 

 the former. The body is compressed, the back generally arched, the 

 mandibles with a palpus, the eyes immovable, and the posterior extremity 

 generally with styliform appendages, and turned beneath. The segment 

 next the head supports the first pair of feet, which correspond to the second 

 pair of foot-jaws. The antennae are slender and project in front. Most of 

 these little animals swim with great vigor, by curving the tail beneath the 

 body and jerking it back. There are fresh water as well as marine species, 

 and they may be seen in most springs of fresh water. Some are found upon 

 the beach under sea weed, or burrowing in the sand, and as they are too 

 narrow to walk, they jerk themselves along whilst lying upon one side. 

 There are two families, each of which contains several sub-families. 



Fam>. 1. GammaridcB. In most of these, the thorax is composed of 



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