124 INTRODUCTION TO CLASSIFICATION. 



2. The PYCNOGONIDA are marine, and also have the abdomen 

 rudimentary ; but the legs, of which there are four pair, are 

 enormously long, and many-jointed. There is a chambered 

 heart, but no respiratory organs. The stomach sends csecal 

 prolongations into the less. The sexes are distinct. 



3. The PENTASTOMIDA have long, annulated, vermiform 

 bodies. Two hooks, representing limbs, are placed upon each 

 side of the mouth at the anterior end of the body. There are no 

 respiratory or circulatory organs, and the sexes are distinct. 



In the embryos, the anterior part of the body is provided 

 with four short, articulated limbs ; while, posteriorly, it is pro- 

 duced into a relatively short abdomen. 



These are parasitic animals. Pentastoma tsenioides, which 

 lives in the frontal sinuses of the dog, is the sexual form of P. 

 dentieulatum, which is found in the lungs and liver of Babbits. 



The Orders of the CRUSTACEA are 



1. The PODOPHTHALMIA. These have twenty somites in the 

 body ; a carapace developed from more or fewer of the thoracic 

 somites; branchiae attached to the thoracic limbs or somites, 

 and a short more or less quadrate heart. With rare exceptions, 

 the eyes are placed upon movable footstalks. 



This order contains the Crabs, Lobsters, Shrimps, and 

 Diastylidse. 



2. In the BRANCHIOPODA, the body is composed of more or 

 fewer than twenty somites. The thoracico-abdominal appen- 

 dages are foliaceous, and resemble the anterior maxillipeds of 

 the Podophfhalmia. The carapace, if present, is altogether 

 cephalic. The heart varies. 



Nebalia, Apus, Branchipus, Daphnia, belong to this order. 



3. The OSTRACODA possess a hard, bivalve, hinged carapace. 

 The total number of appendages, including those attached to 

 the head, does not exceed seven pair, and the thoracic limbs 

 are strong legs. The antennae are similar in form and function 

 to the latter. The abdomen is altogether rudimentary, and there 

 is no heart. To this order belong the genera Cypris and 

 Cythere. 



4. The PECTOSTRACA are Crustacea which are fixed in the 



