Everything Has a History by Haldane J. B. S.;

Author:Haldane, J. B. S.;
Language: eng
Format: epub
Publisher: Taylor & Francis Group
Published: 1951-06-15T00:00:00+00:00

7 Fish

DOI: 10.4324/9781315666709-39

We men belong, together with four-footed animals, birds, fish and so on, to the phylum called Vertebrates, characterised, among other things, by a many-jointed back-bone.

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. It also agrees with us and with some of the echinoderms, and differs from most other invertebrates, as regards the biochemistry of its muscles. Some zoologists take these resemblances more seriously than others. If it is a relative, so probably is a living creature called Rhabdo-pleura which lives a life rather like a tiny sea anemone in the deep sea. And so were a large extinct group of polyps called the graptolites.

Few zoologists doubt that we are related to the tunicates or sea-squirts. When adult they are mostly fixed to rocks, sucking water in through one hole and squirting it out by another after filtering food out of it. But many of them start life as “tadpoles” resembling a vertebrate in a great many respects, and some of these tadpoles do not settle on rocks, but swim throughout their lives. Roughly speaking we may say that the tunicates are related to fish and men as barnacles are related to shrimps and bees. They have become fixed and lost many organs. We have kept moving and gained new ones.

A much nearer relative is the lancelet, Amphioxus, which may be described as a fish without a head. As it is also boneless the relationship is hard to prove from the fossil record. But by great good fortune a few quite ancient fossils of a boneless animal called Jamoytius have been preserved in shale, and are like lancelets but with better developed eyes. So the few who doubted the relationship of Amphioxus to the vertebrates are now fewer.

The most primitive living vertebrates are the lampreys and their relatives. Superficially they look like eels. But they have no paired fins and no jaws. Instead they have a round mouth and horny teeth, with a number of gill slits opening out of their gullets. The most ancient fish whose skeletons we possess resembled the lampreys in having no jaws and no paired fins, but were heavily armoured. They were probably not the ancestors either of modern fish or of men. The next group of fish to appear on the fossil record had primitive jaws and were beginning to develop paired fins. Different lines of them tried one, two and three pairs of fins, and only those with two pairs have left descendants.

The fish which are alive to-day belong to many groups, but only two of them are important. The sharks and rays have skeletons of cartilage, not of true bone, and are primitive in many ways.

They have probably survived because they look after their children better than most other kinds of fish.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.