The alimentary canal begins to be formed by its two opposite extremities. The earliest traces of the oral aperture are perceived nearly at the same time as the labrum, under the form of a small cavity, which becomes continually deeper and deeper. Some short time after wards, and before the appearance of the jaws, we distinguish towards the summit of the ab dominal tubercle, a slight depression which grows rapidly deeper in order to form the anus. About the same period a very delicate and gelatinous-looking membrane begins to be formed between the inner aspect of the middle portion of the blastoderma and the vitellus ; this increases rapidly, and sends prolongations towards the mouth and anus, which soon be come hollowed out into a cavity, and are fi nally converted into two small perpendicular canals. The one of these canals terminating at the mouth is the commencement of the cesophagus and stomach ; the other,with which the anus is soon found to be in connexion, is the rudiment of the intestine. The rest of the membrane in question is obsekved to extend rapidly and at length completely to envelope the vitellus. At this epoch of the develop ment of the embryo, the sac thus formed covers the blastoderma, incloses the yolk, and towards its lower part presents two funnel-like portions by which it is made to communicate with the gastric and intestinal portions of the digestive canal, the formation of which we have just had occasion to speak of. These two portions of the digestive canal as they increase .in size approach one another ; the rest of the sac folds inwards upon itself, and diminishes more and more in size until it disappears entirely, and the stomach and in testine form one perfectly continuous tube. At the point where the intestine is connected with the sac inclosing the yolk, two small thickenings are seen, which by-and-by acquire the form of appendages and become covered with little warty-looking enlargements ; this is the liver beginning to be formed. The enlarge ments of which we have spoken constitute its lobuli, and these slowly divide into a mul titude of long slender vessels.
The heart begins to be developed about the same time as the intestinal canal. It makes its appearance towards the dorsal part of the body, a short way above the commencement of the abdomen, and shows itself at first under the guise of a small pyriform cavity hollowed out of a membrane supplied by an inner la mina of the blastoderma. The arteries begin to show themselves towards the same period in the substance of this same blastodermic lamina, and in the beginning present neither ramifications nor any communication with the heart.
We have already spoken of the develop ment of the apparatus of respiration and of that of the nervous system at such length as to render it unnecessary to enter farther upon these parts of the subject here.
The greater number of the Crustacea do not escape from the membranes of the egg until they have attained such a perfect state of develop ment, that they possess the whole of the organs they will ever exhibit, and have attained a form which differs but little from that which is to distinguish them when arrived at matuiity or become adult. The case, however, is different as regards some of these animals; these are born in some sort prematurely, and only attain their distinctive formation after their exit from the egg. The changes which they undergo between the term of their birth and that of their perfect growth are sometimes so great that they are every way deserving of the name of metamorphoses.
These changes, whatever their amount, may depend on the following circumstances :-1. the continuation of the normal work of development, which has not been completed in the ovum ; 2. the unequal growth of different parts of the body ; and, 3. the atrophy and complete ulti mate disappearance of certain parts.
It is among the lower Crustaceans that this kind of premature birth takes place most fre quently : thus the sugient Crustaceans and the Entomostraca quit the membranes of the ovum at a stage of development which corresponds with one of the earlier of those under which the Decapoda present themselves to our notice ; they are all of an oval figure, and only appear provided with a very limited number of styli form extremities. The common Cyclops, for
instance, does not show the posterior part of the body at the time of its exclusion from the ovum, although this subsequently forms an elongated tail; it is nearly spherical at first, and is provided with no more than two anterince and four extremely short feet. It continues in this state till the fourteenth day, when a small projection makes its appearance from the hinder part of the body ; on the twenty-second day it acquires a third pair of extremities, and on the twenty-eighth day it changes the tegu mentary covering of its body.* Several Edrioph thalmians are also born before they ha.ve ac quired the whole of their extremities ; but we know of no instance of the appearance of one or more pairs of extremities after exclusion from the ovum among the superior Crustaceans.
The changes of form which take place in parts already existing, and which depend on the unequal rates of increase with which the different parts of the animal approach their final state of development, are often very con siderable, and commonly tend to occasion peculiarities of conformation in the adult, which distinguish it from allied species, and imprint upon it the character proper to the tribe, genus, species, and even sex to which it belongs. These implicate one part in one, another in another ; liere it is the thorax which grows more rapidly than the abdomen and greatly preponderates ; there it is the abdomen which, smaller at first than the thorax, increases in dimensions, and finally exceeds it in size : in other instances, again, the phenomenon of extraordinary growth is displayed in certain extremities, or even in certain articulations of these extremities, which follow differences in the proportions of the body and in the forms of its different parts. These differences contribute in general to in crease the dissimilarity which already exists between the different segments of the body, and may therefore be regarded as a sequence in the general tendency of these animals to become more complicated in their structure in propor tion as they rise in the series to which they belong, or in the course they have to run in order to attain their perfect state.
To conclude : the modifications depending on the atrophy and the disappearance of certain parts with which the embryo is provided, tend also to individualize in a greater and greater degree the animals which experience them. As an instance of this phenomenon we may quote the disappearance of the eyes in certain Haustellate and certain Edriophthalmian Crus taceans, and that of the greater number of the extremities in a great many of the Lernew. The Dromim, among the Decapod anomoura, have also presented us with an instance of changes, analogous in their nature and in their consequences; for among the young animals the abdomen terminates in a caudal, fan shaped fin, as among all the Macroura and a great many of the Anomoura; but with the advance of age, the lateral laminw of this organ disappear, and the abdomen then termi nates very nearly as it does in the Brachyura.
It is among the Crustacea which are born in the most imperfect state, and which conse quently have the greatest number of changes to undergo, that the young animals bear the greatest resemblance to one another. The anomalies of conformation encountered among these Crustacea do not in general s.how them selves till the latter periods of their growth.
The length of this article (already, perhaps, too great) does not allow of our pausing longer. on this subject, and we shall only add that the evolution of the Crustacea is one of the points in the history of these animals which appears to promise the most interesting and -important series of facts to whoever will devote himself to the comparative and extended investigation of the subject.*