CYST. 



787 



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 have 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 ; here 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 Lerneae. 

 The Dromiae, 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 laminae 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 



* J urine, Histoire dcs Monocles. 



these Crustacea do not in general show them- 

 selves till the latter periods of tlu-ir Drouth. 



The length of this article (already, perhaps, 

 too great) does not allow of our pausing longer 

 on this subjeet, and we shall only add that the 

 evolution of the Crustacea is one of the points 

 in the history of these animals which ;].; 

 to promise the most interesting and important 

 series of facts to whoever will devote himself to 

 the comparative and extended investigation of 

 the subject.* 



BIBLIOGRAPHY. Besides the references at the 

 bottoms of the preceding columns, sec Suchow, Anat. 

 physikalisch Untcrsuchung ueber Insckten und Crus- 

 tenthierc, 4to. Heidelb. 1818. Porting, De cancri 

 fluviatilis partibus genitalibus, Miscel. Acad. Natur. 

 Curios. Dec. 2, An 1687, p. 48. Gesecke, DC 

 cancri astaci quibusdam partibus, 4to. Gotting. 

 1817. Kvhler, Obs. nonnulas anatomicas, &c. et 

 in systema vucorum cancri astaei, 8vo. Tubing. 

 1811. Herbst, Naturgeschte der Krabben und 

 Krebse, 3 Th. 4to. Bcrl. 1782-1800. Miiller, En- 

 tomostraca sen insecta testacea, &c. 4to. Koprnh. 

 1785. Ramdokr, Beytrage zur Naturgeschicbte 

 einiger deutschen Monoculusarten, 8vo. Halle, 

 1805. Hunter, Catalogue of the Hunterian Col- 

 lection in the Museum of the Royal College of 

 Surgeons Comparative Anatomy and Physiology, 

 4to. 1831-5. 



(H. Milne Edwards, j 



CYST. Kystus, (KVO-TK;, bladder). Certain 

 membranous investments, of various forms, 

 though commonly spheroidal, being shut sacs, 

 and developed in the midst of other tissues, 

 have obtained the name of cysts. 



Up to the present moment the study of cysts 

 is so little advanced that we can scarcely dis- 

 cover any researches which would appear to be 

 founded upon the observation of nature. Whilst 

 so much attention has been devoted to the in- 

 vestigation of many departments of patholo- 

 gical anatomy, it is difficult to understand why 

 this very interesting subject has been compara- 

 tively neglected. The singularity of the cir- 

 cumstance is not lessened by the reflection that 

 the rules of therapeutics ought to vary with the 

 character of these sacs, and that, consequently, 

 the anatomical study is of first-rate importance 

 in enabling us to proceed rationally in the treat- 

 ment of these extraordinary products of the ani- 

 mal economy. 



In describing these organs, two modes have 

 commonly been employed ; the one, to con- 

 sider them with reference to the product they 

 contain ; the other, with reference to their pro- 

 per structure. It is not our intention to adopt 

 either of these methods of considering the sub- 

 ject, and for the following reasons: it is de- 

 monstrable that cysts which are identical in 

 texture frequently envelope totally different 

 products, and also that the products and the 

 cysts are susceptible of transformation to an 

 almost indefinite extent ; and as neither method 



* See on this subject the observations of Rathke 

 already quoted ; those of Thompson, " the Meta- 

 morphoses of the Crustacea;" our own " Rc- 

 cherches sur les changemens de forme quo les 

 Crustacea subissent dans le jeune age." (Annalrs 

 des Sciences Naturelles, torn. xxx. and 2ne scric, 

 torn, iii.) and the inquiries of Nordmann in his 

 Mikrographische Bcitrage, &c. 2tes Heft. 



