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 crawUng 

 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 air 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 



