CRUST A < 



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at these extremely simpk appendage* into organ* of a complex 

 towtwre, entirely distinct from the extremities, hut .till entirely 

 sternal ; 4, tad bully, that during which the branchiie link inwards, 

 a*>d bwnme lodced in a cavity especially adapted for their reception, 

 and provided with a particular apparatus destined to renew the water 

 necessary to the maintenance of respiration. 



If we now turn to the examination of the apparatus of respiration 

 in the different groups in which it exhibits important modifications, 

 we shall, in the series of Crustaceans, encounter permanent states 

 analogous to the various phase* through which we hare just seen the 

 apparatus pasrinr in the moat elevated, *"'"*'* of the class. And in 

 hot the Ant period which we have particularised above in the 

 embryonic life of the Decapod i* exhibited in the permanent condition 

 of some inferior Crnstaoeana, in which not only are there no special 

 organs for respiration, but in which none of the appendices occur 

 with such im^lfrBI^P* of structure as would fit them to become *nb- 

 for the branchiir, in which, consequently, the process of 

 that ia, the aeration of the blood, appears to take place 

 over the surface of the body at large. The greater number of the 

 Hanstellate (Vwtatto, of the I'ulomo^raca properly so called, of the 

 < o/Kpmfa. and even of the Pkyllotomata, appear to belong to this 

 type of organisation." 



The branchial character is so inherent in this class, that it is pre- 

 served even in certain species that live on the land. The Land-Crab 

 (Cxrsrrimul, for example, would die if long immersed in water ; but 

 this, as well as other land Crustaceans, require* a certain degree of 

 moisture to enable the branchiie to act, and accordingly they never 

 remove far from damp situations. 



Much light ha* been recently thrown on the anatomy of the nervous 

 system ana senses in the trust area. 



The principles derived from the study of the gradual evolution of 

 tb* nervous system in the common I'rawfi-h are 1. Tbe isolated 

 formation of the nervous centre*, independently of one another. The 

 ulterior junction of the organ* constitutes the law of centripetal 

 development of M. Serres. 2. A tendency to conjunction by a motion 

 transversely. 3. A second motion in the line of the axis of the body, 

 producing a final concentration of a greater or lea* number of nervous 

 centres, originally independent of each other. 



The first of these condition* is well seen in Talilnu. On each side 

 of the mesial line in thi* genus i* a chain of ganglion*, conjoined by 

 nsjvou* centres of simple construction, flattened, and somewhat 

 loatnge-like in their outline. Thirteen pain of these correspond to 

 the thirteen segment! of the body, the two nuclei of each pair com- 

 iMi*W together in the same way that each pair is connected with 

 i antecedent and succeeding pair, by means of medullary cord* in 

 the first case, and longitudinal cord* in the second. Each of these 

 pain in all eesrntisli is a counterpart of the other, the cephalic gang' 

 lion, which sends branches to the antenna: and eyes, not excepted. 

 In Pltfllotoma the tendency to centralisation is more obvious, and in 

 Cjmoikof the union of the medullary nuclei is accomplished, the 

 approximated chains forming a single longitudinal aerie* from head 

 10 ML 



In the type*, a* might be expected, the centralising system is per- 

 fected by the actual conjunction of the nuclei. This subject has been 

 fully treated ly Rathke, Audouin, Milne-Edward*, and Newport 

 Mr. Newport's excellent and instructive paper ' On the Nervous Sys- 

 tem of the S,J,injc limutri of Linnams' (' Phil. Trans.' part ii., 1834), 

 including beautiful illatrations of the nervous system of the Lobster, 

 and showing ite identity in principle with that of the Sphinx, may be 

 consulted with advantage. 



Tb* conclusion formed by M. Milne-Edwards in his ' Histoire ' is, 

 that " tb* nervous system of the Crustacea consists uniformly ol 

 medullary nuclei (ganglions), the normal number of which is the 

 > as that of the members or rings of the body, and that all the 

 iftcatiooa encountered, whether at different periods of the incuba- 

 tion or in different species of the series, depend especially on the 

 approximation, more or loss complete, of these nuclei (an approxima- 

 tion which take* place from the sides towards the median line, as well 

 as in the longitudinal direction), and to an arrest of development 

 occurring in a variable number of the nuclei." 



Mr. Newport appears to have been the first who pointed out the 

 double gangliouic chain in the Lobster, a* being composed of two 

 orders of fibres, forming distinct and superposed fasciculi or columns, 

 dsnknstiil by him column* of sensation and of motion. 



Tbe highest degree of nervou* centralisation ia found in Main, 

 according to M. Milne-Edwards, who lay* down the following princi- 

 pies, the result of the experiment* made by himself and M. Audouin, 

 and his deep and elaborate Investigation of the subject : 



" 1 Tbe nervous system is the system which entirely preside* o 

 the motion* and motion*. 



" S. Tb* nervous cord* are merely the organ* of transmission of the 

 nsation. and of volition, and It i* in the ganglion* that the power 

 of perceiving the former and of producing the latter resides. 

 '*&* asperated from ha nervous centre speedily loses all motion and 



ite 



V The whole of the ganglions have analogous properties ; the 



< Untuning motion* and receiving mutations exists in each 



rihew organ* j and the action of each is by so much the more iudc- 



pendent as ite development i* more isolated. When the ganglionic 

 hain ia nearly uniform through ite whole length, it may be divided 

 without the action of the apparatus being destroyed in either portion 

 :bus isolated, always understood, that both are of considerable size, 

 Because, when a very small portion only is isolated from the rest of 

 Jie system, this appears too weak, as it were, to continue ite func- 

 tions, so that sensibility and contractility are alike speedily lost Hut 

 where one portion of the ganglionic chain has attained a development 

 very superior to that of the rest, ite action becomes essential to the 

 integrity of the functions of the whole. 



"It must not be imagined, however, from this that sensibility and 

 the faculty of exciting muscular contractions are ever completely 

 concentrated in the cephalic ganglion*, and it seem* to us calculated 

 to convey a very inaccurate idea of the nature and function* of these 

 ganglions to speak of them under the name of brain, as the generality 

 of writer* have been led to do, seduced by certain inconclusive analo- 

 gies in point of form and position. 



" It is nevertheless to be remarked, that in these animals an obscure 

 tendency to the centralisation of the nervous functions is observable 

 in the anterior portion of the ganglionic chain ; because if, in the 

 lobster, for instance, it be divided into two portions, as nearly equal 

 as possible, by severing the cords of communication between the gan- 

 glions belonging to the first and second thoracic rings, sensibility, and 

 especially mobility, are much more quickly lost in the posterior than 

 in the anterior half, and this diaproportion is by so much the more 

 manifest as the division i* performed more posteriorly ; still there i* 

 a great interval between this first indication and the concentration of 

 the faculties of perception and of will in a single organ the brain 

 of which every other portion of the nervous system then becomes a 

 mere dependency." (' Cyclo. of Aunt and Phys.') 



The sense of sight is possessed by the whole of the class at some 

 period of tin ir lives, and in the great majority the organ is of a 

 highly complicated structure. The parasitic Cnutacru, which 

 undergo a kind of metamorphosis, possess eyes in the early stage of 

 their existence, though they are subsequently obliterated ; but the 

 great mass of Crustaceans are gifted with the power of distinguishing 

 objects through the medium of light from their birth to their death. 

 The different forms presented by the visual apparatus are as 

 follows : 



Smooth or Simple Eyes. These consist of a smooth rounded 

 transparent cornea, being a modification of the tefrumentary mem- 

 brane, immediately behind which tend in contact with it is the 

 crystalline lens, generally spherical, and behind this hut and in con- 

 tact with it is a mass of gulntino. which performs the function of the 

 vitreous humour, and touches the extremity of the optic nerve. A 

 thick deep-coloured pigment envelops the whole, and lines the inner 

 surface of the eye-globe up to the point at which the transparency of 

 the cornea begins. Limvlut (Molucca Crab, King-Crab) affords an 

 example of this kind of eye. The simple eyes have never been found 

 to exceed two or three in number. 



Intermediate Eyes. Nrbalia, Branchipui, and Daphnia present us 

 with the first modification of a visual structure, intermediate as it 

 were between the simple and the compound eyes. In this organisa- 

 tion the cornea U still undivided externally, but a number of small 

 crystalline lenses and vitreous humours, each in ite separate pigmen- 

 tary sac and terminating in immediate contact with the optic nerve, 

 presents an eye consisting of a conjunction of several etemmata or 

 simple eyes under a common cornea A put [BuiocoLus], besides its 

 pair of simple eyes, has also a posterior compound pair. The second 

 modification, which is to be found in the Edriophthalmians (A mpki- 

 that, for instance), brings us still nearer to the truly compound form 

 with distinct facets. Two transparent lamina; form the cornea in 

 these Crustacean* : the external is smooth and undivided, the internal 

 divided into a variable number of hexagonal facet*, each with a 

 distinct cornea, which are superposed upon the conical crystalline lens, 

 which ia an ingredient in compound eyes properly so called. 



Compound Eyes. The external and internal membranes, the junc- 

 tion of which forms the cornea, present simultaneously the division 

 into facets, each of which forms anteriorly an ocular compartment 

 Unlike the facets in the eyes of insects, which are always hex: 

 these present various figures in different Crtutacea. I' 

 (ialalhta, the common Crawfish, Ac., for example, they are square : in 

 Paguriu, Squilla, the Crabs, tc,, they are hexagons. The erystnllinn 

 humour that succeeds them immediately, is, according to M. Milm- 

 Edwards, "of a conical form, and is followed by a vitreous humour 

 having the appearance of a gelatinous nlnnicnt. adhering by its base to 

 the optic nerve." Each of the columns thus formed is moi 

 lodged within a pigmentary cell, which likewise covers the bull> of 

 the optic nerve. " Rut the most remarkable circumstance is, that the 

 large cavity, within which the whole of these parallel columns, every 

 one of which in in itself a jwrfect eye, are contained, is closed poste- 

 riorly by a membrane, which appears to be neither more nor less than 

 the middle tegutnentary membrane pierced for the passage of the 

 optic nerve, so that the ocular chamber at largo results from the sepa- 

 ration at a point of the two external layers of the general envelope." 

 . . . . " The most remarkable modification of facetted eyes con- 

 sist* in the presence of a kind of supplementary lens, of a circular 

 shape, and net, within the cornea in front of each proper crystalline 



