GENERAL DESCRIPTION 327 



Five pairs of appendages are present in the head, the first antennae, 

 second antennae, mandibles, first maxillae, and second maxillae. Of these, 

 the first pair of antennae (antennules) differs from all the other append- 

 ages of the body in not being typically biramose; they are not, however, 

 necessarily simple, but the distal portion of the appendage is frequently 

 split into two, three, or more branches, called flagella (Fig. 614,1). The 

 mandibles are short, stout appendages, fitted for biting, and may bear a 

 sensory palp which is the modified endopodite, the exopodite being want- 

 ing. The two pairs of maxillae are usually delicate structures whose 

 function is probably chiefly sensory. 



The number of thoracic appendages varies greatly among crustaceans. 

 The smallest number of pairs (2) is found among ostracods and the 

 largest number (60) among the Apodidae. The abdominal appendages 

 are wanting in the Entomostraca and in the youngest larval forms of 

 most Malacostraca. In adults of the latter group, however, these are 

 present on all the abdominal segments except the last one (telson). 



The cuticula of crustaceans is shed periodically. In the smaller 

 species it is very delicate and the animal is often quite transparent. In 

 the large Malacostraca it contains calcium carbonate as well as chitin and 

 is very hard and thick : it is such animals which have given the group the 

 name Crustacea. 



Internal Structure (Fig. 623). The digestive tract is in most crus- 

 taceans a straight tube going from the ventrally located mouth to the 

 anus at the hinder end of the body. Tubular livers, often very volumi- 

 nous, are present in most forms; salivary glands are absent. The ex- 

 cretory organs consist of a pair of tubular glands, the kidneys, which 

 open to the outside in the neighborhood of the mouth. The respiratory 

 organs are lacking in some of the small crustaceans, the outer surface of 

 the body performing this function. In most of them, however, gills are 

 present, las projections of the thoracic or abdominal appendages or of 

 the sides of the body. 



With the exception of most of the Cirripedia, which are all either 

 sessile or parasitic, all crustaceans are unisexual. Among the PJiyllopoda 

 and Ostracoda parthenogenesis is common. In the lower crustaceans the 

 animal usually leaves the egg as a nauplius larva, 'a minute animal with 

 three pairs of appendages, of which the first pair is uniramose and the sec- 

 ond and third pairs are biramose. With a few exceptions (Peneus, Lucifer) 

 all the higher crustaceans pass through the nauplius stage while still in 

 the egg and are born in some later stage of development. Many of them, 

 as the crayfish, have the form of the adult when born, the entire meta- 

 morphosis having been completed in the egg. 



Habits and Distribution. The sowbugs, land crabs, and a few other 



