786 



CRUSTACEA. 



elongated lamina, the free end of which is 

 turned forwards, and before long advances 

 nearly to the labrum. 



In the space included between the mandibles 

 and the fold formed by the abdominal lamina 

 of the embryo, of which we have just spoken, 

 we now perceive the rudiments of two pairs of 

 jaws and of the first pair of maxillary extremi- 

 ties, then of the second pair of these latter or- 

 gans, and soon afterwards of the third pair. 

 These appendages appear in the same man- 

 ner as the antennae, and in proportion as they 

 are evolved, the fold that marks the origin of 

 the caudal lamina of the embryo recedes from 

 the anterior part of the body; by little and little 

 the basilar portion of the lamina becomes 

 straightened, so as to gain the same plane as 

 the remainder of the blastoderma, whilst its 

 terminal portion continues bent underneath 

 against the former. The five pairs of ambula- 

 tory extremities make their appearance succes- 

 sively in the same manner as the antennae 

 and the oral appendages; the same may be 

 said with regard to the abdominal extremi- 

 ties ; and whilst this formation is going on, the 

 annular divisions of the abdominal portion of 

 the body are observed to be evolved. The 

 carapace at length begins to be formed in 

 the manner already indicated, and the ex- 

 tremities, as they sprout, alter their shapes, 

 and become more and more unlike one 

 another, as they approach the term of their 

 embryotic development. 



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 

 oesophagus 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 obseived 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 maturity 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 antennae 

 and four extremely short feet. It continues 



