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 Sacculina. Fortunately the latter 
appears to live only three years, and when it dies the crab 
resumes its growth. 
B. MALACOSTRACA. 
CHARACTERISTICS.— The Malacostraca include the more conspicu- 
ous and more highly differentiated Crustacea. The number of 
segments and pairs of appendages is constant. With one 
exception, there are nineteen segments, each bearing a parr of 
appendages. The head consists of five, the thorax of eight 
segments. The abdomen has six segments, and ends in 
an wnsegmented telson. The excretory organ usually opens 
on the second antenna, and is called the antennary gland, as 
opposed to the Entomostracan maxillary or shell gland. The 
Nauplius larva, so characteristic of the Entomostraca, is rare 
in the Malacostraca. Some of them hatch out from the egg 
in the adult condition ; the majority, 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 MALACOSTRACA include three orders : 
1. LEPTOSTRACA. 
2. THORACOSTRACA. 
3. ARTHROSTRACA. 
