TYPE CRUSTACEA. 417 



a number of the limbs also in females bear lamellae Avhich 

 may enclose a brood-pouch. The three anterior abdominal 

 limbs are biranious and serve for swimming, while the three 

 posterior ones, also biramous, are frequently directed back- 

 wards and serve as springing organs, the springing powers of 

 Orchestia having gained for it the name of the Beach-flea. In 

 Caprella, which crawls about over colonies of Hydroids and 

 Polyzoa, and Cyamus, which is parasitic upon the skin of 

 whales, the abdomen becomes almost rudimentary and is des- 

 titute of appendages. 



The heart (Jit) lies in the thoracic region in the anterior 

 five or six segments and possesses from one (Corophium) to 

 three ostia. It is prolonged into an aorta at either end. In 

 connection with the mid-gut portion of the digestive tract, in 

 addition to the four so-called Irver-cseca (I) is a pair of gland- 

 ular cseca which seem to be excretory in function and have 

 been termed Malpighiau tubules (nit). An antennary gland 

 occurs, but the shell-gland is apparently unrepresented in 

 adults. 



Development of the Crustacea. The majority of the Crus- 

 tacea pass through a more or less complicated series of 

 metamorphoses, the larval forms being highly suggestive 

 when studied from the phylogeuetic standpoint. A few forms, 

 especially those inhabiting fresh water, abbreviate their de- 

 velopment considerably, so that the young animal when it 

 leaves the egg practically may differ from the parent only in 

 size (Cambarns), and among the higher forms the development 

 is generally abbreviated to the extent that a greater or less 

 number of the larval stages, characteristic of lower forms, are 

 passed through while the young animal is still within the egg- 

 membrane, only the final stages being free-swimming. 



Throughout the Entomostraca the first larval form which 

 hatches from the egg is termed the Nauplius (Fig. 191) and 

 differs markedly from the adult, chiefly, however, in the small 

 number of appendages it possesses. The body in typical 

 forms shows no trace of segmentation and possesses a single 

 1 median eye generally x-shaped. But three pairs of limbs 

 are present, which become transformed later into the anten- 

 nules, antennae, and the mandibles of the adult. The Naupliar 



