If only Sea-scorpions had not unfortunately died out, how interesting it would be to watch them alive, and to see exactly what use they would make of their long bodies, tail-flaps, and tail-spikes! Were they nocturnal in their habits, wandering about by night, and taking their rest by day? Such questions, we fear, can never be answered. But their large eyes would have been able to collect a great deal of light when the moon and stars feebly illumined the shallower waters of the seas of Old Red Sandstone times; and so there is nothing to contradict the idea. Now, it is an interesting fact that young crabs, soon after they 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 _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 favour of such a change. It will thus be seen that our name Sea-scorpions is quite permissible. Hugh Miller described some curious little round bodies found with the remains of the Pterygotus, which it was thought were the eggs of these creatures!