264 
THE LIAS AMMONITES. 
so that we may be able to take an onward step towards a biglier and better knowledge 
of the natural history of these wonderful extinct polythalamous Cephalopods. 
Morphology has taught ns that all animals pass through certain changes of form 
and structure between their escape from the ovum and the period they attain their adult 
condition, and that the nature and extent of these changes differ in different classes, 
families, and genera of the animal kingdom. 
In the Acaleplm a most remarkable metamorphosis is observed in the Medusida, 
which was first described by M. Sars,^ a Norwegian naturalist, and afterwards studied in 
1846 by the late Professor John Reid and myself on specimens taken in St. Andrew's Bay, 
N.B., which showed the marvellous changes Aurelia aurita passes through in the course of 
its evolution. The egg produces an ovoid embryo, which is covered with a ciliated epithe- 
lium and swims freely about for a time like an Infusorian. After some days it fixes 
itself by a disk, and at the upper side of the small body a depression is seen with 
four corners, which gradually become elongated, and by degrees are transformed into 
tentacles. These rapidly multiply until the whole of the upper margin is covered 
with them, then transverse constrictions at regular distances apart are seen on the body 
stem, first appearing above, and afterwards extending downwards towards the disk. 
These constrictions grow deeper and deeper, and at the same time the edge of each 
segment begins to be serrated, so that the animal presents the appearance of a pine-cone 
surmounted by a tuft of tentacles. The separation goes on constantly, until at last the 
divisions are united only by a very slender axis, the whole resembling a pile of saucers 
placed one within another at a certain distance apart. The divisions are now ready for 
separation; the upper ring first disengages itself, and then the others follow in succes- 
sion. Each separate segaient now continues its morphology alone, and afterwards 
becomes a complete Medusa ; whilst the original body-stem remains, and, it is said, 
produces a new colony. On this subject consult also Sir J. G. Dalyell's Memoirs. 
The morphology of the JSchmodennata, in the Ophiuroidea, Asteroidea, and Echi- 
noidea, has been so carefully observed by Sars, Agassiz, Professor J. Miiller, Krohn, 
and others, that we are now well acquainted with the successive forms these Radiates 
pass through before they attain their adult state, changes, — in fact, so numerous and 
remarkable that it was only after the most ample verification by difierent independent 
observers that the conclusions first published by the late Professor J. Miiller were 
admitted as real biological facts. 
The morphology of the Artlirojjoda, comprising the classes Insecta, Myriopoda, 
Arachnida, and Crustacea, affords abundant illustrations of the great law of change. In 
the Crustacea, the lower forms of Entomostraca, found in every freshwater pool, are 
interesting and instructive examples as microscopic objects, and Apus, DapJmia, Cyclops, 
and Cldrocephalm, may be mentioned as convenient subjects for study. The eggs of 
Crabs and Lobsters among marine forms, and Astacus fluviatilis from freshwater 
streams, afford good objects for watching the morphology of the Malacostraca. 
1 Sars, ' Isis,' 1833, p. 224, taf. x, fig. 4 ; and ' Beskrivelser og Jngtag,' kc, p. 16, pi. Hi. 
