Many of the Rotifers have a pair of ruby-red eyes, and in some of them there is a minute crystalline lens overlying the red sensitive spot, which receives the fibres of the optic nerve coming from the brain–one on each side. It is almost incredible that so minute a creature–often only the one-fiftieth of an inch long when full grown–should have a nervous system and special organs of touch (sensory hairs) as well as eyes, and on the other hand muscles running from one attachment to another and called into activity by nerves connected with this same central brain. The pair of branched tubes, which end internally in flickering "flame-cells" and open externally far back at the vent, are kidneys. Similar tubes called "nephridia" or little kidneys are found in many of the smaller animals; the earthworm has a pair in each ring of its body. There is little doubt that the wheel animalcules are related in pedigree to the primitive ancestors of the marine segmented or annulate worms, which also gave rise to the ringed leg-bearing jaw-footed creatures with hard skin, called Crustacea, Arachnids and Insects (the Arthropods). The wheel-apparatus or cilia-fringed discs of the Rotifer is seen in the young stages of many marine worms, and also in the young of marine snails, known as the "veliger"–"velum" or "sail" being the name given to the wheel-apparatus of the young snails (see the drawing on p. 181). There are very minute marine annulate or segmented worms (Dinophilus and others), which come near to the Rotifers in many features, whilst the ringed or segmented character of the body is obvious in the common wheel animalcule. The Rotifers are so small that they are built up of very few "cells" or nucleated units of protoplasm. Many of them are of smaller size than some of the big infusorian animalcules, which consist of a single cell. The Rotifers are probably a dwindled pygmy race descended from ancestors of ten or a hundred times their linear measurement. It is an important fact that in the possession of a toothed gizzard, in the hard body-case or cuirass of some kinds, and in Pedalion's rapidly-moving legs or paddles, fringed with plumose hairs and moved by that peculiar variety of muscular tissue which is called "striped muscular tissue," the wheel animalcules give evidence of relationship to the Crustacea–that is to say, it appears to be probable that they were derived from the common ancestor of marine worms and Crustacea before those two lines of descent had diverged.