6 A HISTORY OF RECENT CRUSTACEA 
to run a few steps into the ocean and out again. Witha 
similar effort of the scientific imagination, the illustrious 
Erasmus Darwin, when a schoolboy, excused himself for 
eating roast goose during Lent by the scriptural axiom 
that ‘all flesh is grass,’ and the goose therefore a species 
of vegetable. 
So far as the name Crustacea implies a covering of any 
considerable toughness, it is little applicable to some of 
the parasitic members of the class, but in general much 
more confusion than advantage follows from the dis- 
placing of long-established names in the effort after 
absolute accuracy. If we are never to use the scientific 
designation of a group unless it exactly applies to all the 
members of it, then what is to be done, one writer 
rather maliciously asks, in the case of the species called 
Homo sapiens ? 
A general though not a complete agreement prevails 
in regard to the externa! boundaries of the crustacean class. 
The proper mode of subdividing it and the arrangement 
of the subdivisions are subjects still open to much dis- 
cussion and dispute. Any final decision depends upon 
questions of genealogy which have yet to be answered. 
Jn the mean time four sub-classes may be accepted, under 
the names Gigantostraca, Malacostraca, Entomostraca, and 
Thyrostraca. The Gigantostraca, or giant-shells, are the 
oldest in known lineage, and, as the name implies, fore- 
most in the average of magnitude. They seem to be 
tending to speedy extinction. The Malacostraca include 
forms highest in development and of most direct value to 
mankind. The Entomostraca probably surpass the rest 
in multitude of individuals, if not also of species, but are 
the smallest in average size. The Thyrostraca, commonly 
called Cirripedia, though they fall short of the Entomostraca 
in numbers, excel them in bulk, and are even more remote 
in outward appearance from any general idea of a crusta- 
cean, such as the better known malacostracan lobster, or 
the crab fish, might suggest. ; 
The Greek word Malacostraca, meaning soft-shelled 
animals, is practically equivalent in sense to the Latin 
