But the annelides also are liable to many persecutions. The fishes are perpetually at war with them; and when an imprudent annelide quits its hidden lurking-place, or is uncovered by the motion of the waves, it may reckon itself fortunate, indeed, if it escapes the greedy teeth of an eel or a flat-fish. It is even affirmed of the latter, as it is of the whelks, that they know perfectly well how to dig the annelides out of the sand. The sea-spiders, lobsters, and other crustacea are the more dangerous, as their hard shells render them perfectly invulnerable by the bristling weapons of the annelides. While the greater part of these worms lead a vagrant life, others, like secluded hermits, dwell in self-constructed retreats which they never leave. Their cells, which they begin to form very soon after having left the egg, and which they afterwards continue extending and widening according to the exigencies of their growth, generally consist of a hard calcareous mass; but sometimes they are leathery or parchment-like tubes, secreted by the skin of the animal, not however forming, as in the mollusks, an integral part of the body, but remaining quite unconnected with it. Thus these tubicole annelides spend their whole life within doors, only now and then peeping out of their prison with the front part of their head. As they lead so different a life from their roaming relations, their internal structure is very different, for where is the being whose organisation does not perfectly harmonise with his wants? Thus, we find here no bristling feet or lateral respiratory appendages; but instead of these organs, which in this case would be completely useless, we find the head surmounted by a beautiful crown of feathery tentaculæ, which equally serve for breathing and the seizing of a passing prey. Completely closed at the inferior extremity, the tube shows us at its upper end a round opening, the only window through which our hermit can peep into the world, seize his food, and refresh his blood by exposing his floating branchiæ to the vivifying influence of the water.