There are a number of groups of animals, some of which are fairly surely, others more doubtfully, related to the vertebrates. A worm called Balanoglossus has a rod resembling the horny rod which precedes the backbone in our development, and slits in its gullet like the gill slits of a fish. The more modern type of bony fish, though they are more advanced than the sharks in many ways, usually lays great numbers of small eggs. So there is a huge infantile mortality, and very few young fish live to maturity. There are still a few living fish which are fairly like those which went ashore in the Devonian, or Old Red Sandstone Age, and became our ancestors. They have paired fins with a bony axis which is much more like the limb of a four-footed animal than are the spiny-fanned fins of modern fish. Most female fish lay eggs in the water; the male pours sperm over them.