264 THE LIAS AMMONITES. 



so that we may be able to take an onward step towards a higber and better knowledge 

 of the natural history of these wonderful extinct polythalamous Cephalopods. 



Morphology has taught us 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 AcalephcB a most remarkable metamorphosis is observed in the MedusidcB, 

 which was first described by M. Sars,^ a Norwegian naturalist, and afterwards studied in 

 1846 by the late Professor JohnReid 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 segment 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 Echinodermata, in the OpMuroidea, 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 adiilt state, changes, — in fact, so numerous and 

 remarkable that it was only after the most ample verification by different independent 

 observers that the conclusions first published by the late Professor J. Miiller were 

 admitted as real biological facts. 



The morphology of the Artliropoda, 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 Ajms, Dajjhnia, Cyclops, 

 and CJdrocephalus, may be mentioned as convenient subjects for study. The eggs of 

 Crabs and Lobsters among marine forms, and Astacus fluviafilis 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 Jagtag,' &c., p. 16, pi. iii. 



