Among fishes metamorphosis is comparatively rare, because both young and adult swim in the water and get their food in much the same way. The best examples are the flatfishes (Hetero somata), in which the adult swims lying on one side. The eye belonging to this side is twisted on to the upper side; and the mouth is distorted towards the lower side, for the fish feeds on the animals (worms and molluscs) which burrow in the sand and mud beneath it. The young fry swim upright in the water and the eyes are on opposite sides of the head. When they have reached a certain size they fall to the bottom and rest on one side. Darwin, quoting Malm, says that they make violent effort to twist the lower eye on to the upper side and that as the skull is soft these efforts meet with considerable success. In a com paratively short time both eyes are on the upper side and the metamorphosis has been accomplished.
The lamprey (q.v.) (Petromyzon), is separated by such a deep gap from all fish that it is placed in a different class, Cyclos tomata (q.v.). No true jaws exist; the mouth is surrounded by a circular cartilage and the animal lives by attaching itself by suctional action to a larger animal and then rasping a hole in its victim's flesh by means of horny teeth which are developed on a piston like tongue. The larval form, known as ammocoetes, lives in a totally different manner. The mouth is overhung by a hood like upper lip separated by a lateral cleft from a small straight underlip. The gill-sacs open directly into the oesophagus instead of into a special tube as in the adult. But the greatest peculiarity of the ammocoetes is that the organ from which the lamprey's thyroid gland is later developed is a sac-like structure with a permanent opening into the throat, through which the mucous secretion escapes in the form of a cord. This entangles any minute organisms in the water which the animal swallows. The mucus with its contained food is passed back into the oesophagus whilst the surplus water escapes through the gill openings.
After living like this for two or three years the ammocoetes undergoes a rapid transformation into the adult form. In 1912 the writer received a consignment of "young lampreys" from the Severn. They were about six in. long and half of them were full grown ammocoetes larvae and half young fully meta morphosed lampreys. They were equal in size.
The animals known as Tunicata (q.v.) are also primitive members of the phylum Vertebrata. The egg develops into a tadpole-like larva with a hollow spinal cord expanding into a brain-vesicle in front. There is a notochord in the tail and the alimentary canal consists of a pharynx, opening above by a mouth in front of the brain, and at the side by a pair of gill pouches. The pharynx is succeeded by a loop of intestine leading to an anus high up on the left side. This larva after swimming for some hours attaches itself to the substratum by three ad hesive papillae on the chin beneath the mouth. Metamorphosis occurs within a few minutes. The tail and notochord are cast off. The spinal cord shrivels and the brain vesicle is replaced by a little solid ganglion, whilst the mouth by the elongation of the chin is elevated above the substratum.
When we descend to Invertebrata, we find the classical exam ple of metamorphosis in the life-histories of the Echinodermata.
In this phylum the larva and the adult are so unlike each other that the change from one stage to the other was formerly sup posed to be an alternation of generations. To render our ideas more definite consider the case of the common British sea-urchin Echinus mz7iaris. The larva is a beautiful transparent free-swim ming form with outwardly perfect bilateral symmetry. It has a scoop-like mouth leading into a narrow gullet. This is followed by a globular stomach from which a short straight intestine leads back towards the mouth to end in the anus. The alimentary canal forms a loop in the median plane. The skin is drawn out into four symmetrical pairs of long arms supported by transparent calcareous rods. These arms are covered by cilia—are in fact prolongations of a lobed, ciliated band which crosses in front of the mouth, passes down the sides and then crosses on the under-surface in front of the anus. Behind the ciliated band the larva has a circle of four crescents carrying powerful cilia.
This general form characterizes larval life for six weeks, grad ually increasing in elaboration as the larva grows older. The arms are at first four, then six, and finally eight, and the ciliated crescents are cut off from the ciliated band. Then the larva sinks to the bottom and extends from its left side the first adult tentacles or "tube-feet" which have been formed under a screen of skin. As soon as the larva has thus come to anchor a mar vellous and rapid change supervenes, which, in the case of another species, the writer has seen completed in about half an hour. The long ciliated arms literally melt away, the protoplasm of the skin covering them seems to gather up into drops and flow backwards off the spines into the body. A large part of the fluid in the primary body-cavity is expelled by osmosis into the stomach, and the globular body of the larva becomes compressed into a flat disc which then crawls away as the young sea-urchin. The mouth becomes cut off from the gullet, shallows out and disappears and a new mouth is formed on the left side in the middle of the circle of tentacles. Similar rapid changes accom panied by shrinkage in size and loss of larval tissues characterize the life-histories of other echinoderms, but in no case are these changes accomplished in so short a time as in the case of the sea-urchin. Consider, for example, the life history of the com mon starfish Asterias rubens. Its larva shows a general similarity to the larva of the sea-urchin. But this larva, the bipinnaria, differs from the sea-urchin larva in two important points: first, its paired arms are more numerous and are unsupported by cal careous rods, and secondly it possesses a long forehead or preoral lobe in front of the mouth, over which a loop of the ciliated band bends back. When its free-swimming life terminates it attaches itself to the substratum not by the adult tentacles but by a sucker which is developed at the apex of this preoral lobe. This lobe is thus converted into a stalk and this attached stage lasts a week or ten days, during which the adult organs are developing at the hinder end of the larva, and the stalk is reduced to a mere knob. The starfish then wrenches itself loose and walks away.