Another very singular Crustacean, known as _Apus_, bears a curious superficial likeness to the King Crab (_Limulus_), having a large back-shield and a long tail. This little creature, a giant compared with his nearest relations, is an inhabitant of wayside ponds and ditches. Thousands of females may be taken for years in succession without the advent of a single male. Then, for some strange reason which we cannot even guess at, males appear. Like its freshwater cousin, the Fairy Shrimp, _Apus_ can withstand drought: its favourite haunts may be transformed into sun-baked hollows, but with a heavy fall of rain and a few hours’ soaking the eggs left by dead females develop, and once more the pool and its inhabitants are established again. Having regard to the extraordinary vitality of these small creatures, it is curious that they should ever disappear from their favoured haunts. But they do. Not many years ago _Apus_ could be found in abundance in many parts of the South of England. It is now extinct; its last resorts were the ponds at Hampstead: now one may search in vain for them. “No British specimens,” remarks Dr. Caiman, a great authority on the Crustacea, “had been recorded for over forty years, and the species was believed to be extinct in this country, when it was found in 1907 by Mr. F. Balfour Browne in a brackish marsh near Southwick, in Kirkcudbrightshire.” These had probably developed from eggs accidentally transported by some bird from the Continent. The extinction of the race throughout the British Islands can only be attributed to the too long absence of males, and the consequent inability to restore vigour by the more normal method of reproduction by sexual congress. Among the Rotifers the little Wheel-animalcules exhibit an even greater vitality, for not only can their eggs withstand prolonged desiccation, but in some the body of the animal survives even harsher treatment. If specimens be enclosed within a chamber containing a little sand or moss the contents may be dried over sulphuric acid, or heated up to 200° F., or left to the neglected dust of years, and will yet revive if a little fresh water be added to the sand. Males are rare, and when they do occur are little more than animated receptacles for semen, for they are incapable of feeding, the gullet and digestive tract being reduced to a solid cord. A certain amount of nourishment, however, may be absorbed through the delicate body wall.