MORE ABOUT WHEEL ANIMALCULES 171 



lying 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 measure- 



