32 EXTINCT MONSTERS. 
are hatched, have long bodies somewhat similar to those of our 
Sea-scorpions, with a head-shield under which are their jaw-feet, and 
then a number of free body-rings without any appendages. These 
end in a spiked tail. As the crab grows older, he ceases to be 
a free-swimming animal—for which kind of life his long body 
is well suited,—tucks up his long tail, and takes to crawling 
instead. Thus his body is rendered more compact and handy 
for the life he is going to lead. Lobsters, on the other hand, can 
swim gently forwards, or dart rapidly backwards. ‘Thus we see 
that the ten-footed crustaceans of the present day are divided into 
two groups—the long-tailed and free-swimming forms, such as 
lobsters, shrimps, and cray-fishes; and the short-tailed crawling 
forms, namely, the crabs. Now, in the same way, Pterygotus and 
its allies were long-tailed forms, while the king-crabs are short- 
tailed forms. So were the trilobites of old. Hence we learn 
that, ages and ages ago, before the days of crabs and lobsters, 
there were long-tailed and short-tailed forms of crustaceans, just 
as there are now, only they did not possess true walking legs. 
They belonged to quite a different order, called ‘‘thigh-mouthed ” 
crustaceans, Merostomata, because their legs are all placed near the 
mouth ; and, as we have already learned, were used for feeding 
as well as for purposes of locomotion. 
Now, one of the many points of interest in Pterygotus and its 
allies is that they somewhat resemble the crab in its young or 
larval state. To a modern naturalist, this fact is important as 
showing that crustacean forms of life have advanced since the 
days of the sea-scorpions. 
Their resemblance to land-scorpions is so close that, if it were 
not for the important fact that scorpions breathe azr instead of 
water, and for this purpose are provided with air-tubes (or 
trachea) such as all insects have, they would certainly be removed 
bodily out of the crustacean class, and put into that in which 
scorpions and spiders are placed, viz. the Arachnida. But, in 
spite of this important difference, there are some naturalists in 
