ARTHROPODA 279 



it will ultimately appear at the outside. In addition to the 

 globular body, which is eventually pushed outside the skin of 

 the crab's abdomen, the parasite has a short peduncle which 

 passes through into the body of the host and gives off an im- 

 mense network of roots which ramify through all the tissues 

 of the crab, and extend even into its limbs. It is by means 

 of these processes that the parasite absorbs its nutriment. 

 This drain upon the resources of the crab does not seem to 

 affect its health, but its growth is arrested, and as a conse- 

 quence it does not cast its skin, an operation which would 

 naturally be fatal to the Saccidina. Fortunately the latter 

 appears to live only three years, and when it dies the crab 

 resumes its growth. 



B. MALACOSTRACA. 



Chaeaoteristics. — The Malacostraca include the more conspicu- 

 ous and more highly differentiated Crustacea. The numher of 

 segments and pairs of appendages is constant. With one 

 exception, there are nineteen segments, each bearing a pair of 

 appendages. The head consists of five, the thorax of eight 

 segments. Tlie abdomen has six segments, and ends in 

 an unsegmented telson. The excretory organ usually opens 

 on the second antenna, and is called the antennary gland, as 

 opposed to the Untomostracan maxillary or shell gland. The 

 Nduplitis larva, so characteristic of the Entomostraca, is rare 

 in the Malacostraca. Some of them hatch out from the egg 

 in the adult condition ; the majm'ity, however, pass through a 

 complicated metamorphosis, the larva leaving the egg in the 

 form of a Zoaea, which is characterised by the presence of the 

 seven or eight anterior pairs of appendages, a swimming tail, 

 and two lateral compound stalked eyes, as well as a median 

 Nauplius eye. The proctodaeum and stomodaeum form a 

 larger part of the alimentary canal of the adult than is the 

 case in the Entomostraca. 



The Malacosteaca include three orders : 



1. Leptosteaca. 



2. Thoracosteaca. 



3. Aetheosteaca. 



