217 



CRUCIROSTRA. 



CRUSTACEA. 



218 



Sub-Order III. ANGUSTISEPT.E. 



Pouch (silicle) short, laterally compressed, opening with two boat- 

 shaped valves, keeled or winged on the back ; dissepiment narrow, 

 linear, or lanceolate. 



Tribe VII. THLASPIDEJE. 

 Cotyledons accumbeut. 



23. Thlu! S ,l. 



24. Hutchinsia. 



25. Teesdalia. 



26. Iberit. 



Tribe VIII. LEPIDINE.E. 

 Cotyledons incumbent. 



27. Lepidittm. 



28. Captella. 



Tribe IX. SUBULARIE*. 



Cotyledons incumbent, long, linear, curved back above their base ; 

 cells many-seeded. 



29. Sulidaria. 



Trilie X. SENEBHIERE.E. 



Cotyledons incumbent, long linear, curved back above their base ; 

 cells one-seeded. 



30. Senebrwra. 



Sub-Order IV. KUCAMENTACK.E. 



Pouch (silicle) scarcely dehiscent, often 1-celled, owing to the 

 absence of the dissepiment. 



Tribe XI. ISATIS. 

 Cotyledons incumbent. 



81. Itatu. 



Sub-Order V. LOMENTACE*. 



Silicle or silique dividing transversely in single-seeded cells ; the 

 true silique often barren, all the seeds being in the beak. 



Tribe XII. CAKlLixtz. 



Cotyledons accumbeut. 



32. Cakite. 



Tribe XIII. RAPHAXE.K. 



Cotyledons conduplicate. 



33. Cramle. 



34. Raphanua. 

 CRUCIROSTRA. [LOXIAD*.] 



CRUSTA'CEA, Crustace's of the French, Krustenthiere of the 

 Germans, MoAaxiisTfKuca of Aristotle and the ancient Greeks, a 

 class of Articulated Animals, whose external covering is less solid 

 than that of the majority of Testaceous Mollusks, but much 

 firmer and harder than the skin of the Naked Mollusks; and whose 

 conformation is essentially distinguishable from other classes, espe- 

 cially in the circulating, respiratory, and locomotive organs. The 

 Common Crab [CRAB], the Lobster, and Crayfish [ASTACUS], the 

 Common Shrimp [CRANGONID*], and the Water-Fleas [BRANCHIO 

 PODA], may be taken as types of different sections of this family. 



As in many of the Testaceous Mollusks, the skeleton of the Crustacea 

 is external. It is made up of the tegumentary envelope, which, ii 

 gome of the class, always continues soft, but in the greater portion is 

 very firm, forming a shelly case or armour, in which all the soft parts 

 are contained. In the more perfect Crustaceans it is complex. The 

 following description of its component parts is from the pen o 

 M. Milne-Edwards, who, in hia ' Histoire Naturelle des Crustacc's 

 (Paris, 1834, &c., 8vo), and in the article 'Crustacea' in the ' Cyclo 

 paxlia of Anatomy and Physiology (London, 1836, Ac.), has given 

 the most complete view of the organisation of this family. Taking 

 the Brackyura, or Short-Tailed Crustaceans, as his instance of the 

 more highly developed forms of the class in which the complex 

 structure is exhibited, he thus proceeds, " The integument consists o" 

 a corium and an epidermis, with a pigmentary matter of a peculia; 

 nature, destined to communicate to the latter membrane the varioui 

 colours with which it is ornamented. The corium or dermis, a 

 among the Vertebrata, is a thick, spongy, and very vascular membrane 

 on its inner surface it is intimately connected with a kind of serou; 

 membrane, which lines the parietes of the cavities in the Crustacea in 

 the same manner as the serous membranes line the internal cavities 

 among the Vertebrata; these two membranes, divided in the latte 

 order by the interposition of muscular and bony layers, which cove 

 and protect the great cavities, become closely united when thes< 

 layers disappear, as they do in the Crustacea, in consequence of th< 

 important changes that take place in the conformation of the apparatu 

 of locomotion. The corium again, among the Crustacea, is completelj 

 covered on iU outer surface by a membranous envelope unfurnishei 

 with blood-vessels, and which must be held in all respects as analogou 

 to the epidermis of the higher animals. It is never found in th 

 properly membranous state, save at the time of the Crustacea casting, 

 their shell; at this period, it is interposed between the corium am 

 the solid covering ready to be cast off, and has the appearance of 

 pretty dense and consistent membrane, in spite of its thinness. I 



orms, as among animals higher in the scale, a kind of inorganic 

 amina, applied to the surface of the corium, from which it is an 

 xudation. After the fall of the old shell it becomes thicker and 

 ery considerably firmer, owing to the deposition or penetration of 

 Calcareous molecules within its substance, as well as by the addition 

 f new layers to its inner surface. The degree of hardness finally 

 cquired, however, and the amount of calcareous matter deposited 

 within it, vary considerably ; in many members of the class, it remains 

 semicorneous, in a condition very similar to that of the integuments 

 if insects, with which, moreover, it corresponds very closely in point 

 if chemical composition ; in the higher crustaceans, again, its compe- 

 tition is very different : thus, whilst chitine in combination with 

 albumen is the principal element in the tegumentary skeleton of some 

 ipecies, this substance scarcely occurs in the proportion of one or 

 rwo tenths in the carapace of the Decapods, which, on the contrary, 

 contains 60 and even 80 per cent, of phosphate and carbonate of lime, 

 the latter substance particularly occurring in considerably larger 

 >roportion than the former. With regard to the pigmentum, it is 

 ess a membrane or reticulation than au amorphous matter diffused 

 through the outermost layer of the superficial membrane, being 

 secreted like this by the corium. Alcohol, ether, the acids, and water 

 at 212 Fahr., change it to a red in the greater number of species ; 

 )ut there are some species in which it may be exposed to the action 

 of these different agents without undergoing any perc3ptible change. 

 The epidermic layer hardened in different degrees is the part whieli 

 mainly constitutes the tegumentary skeleton of the Crustacea. In its 

 nature it is obviously altogether different from that of the internal 

 skeleton of the Vertebrata; still its functions are the same, and this 

 physiological resemblance has led naturalists to speak of these two 

 pieces of organic mechanism, so dissimilar in their anatomical relations, 

 under the common name of skeleton. The tegumentary skeleton of 

 the Crustacea consists, like the bony skeleton of the Vertebrata, of a 

 great number of distinct pieces connected together by means of 

 portions of the epidermic envelope which have not become hardened, 

 ,n the same way as, among the higher animals, certain bones are 

 connected by cartilages, the ossification of which is only accomplished 

 in extreme old age." 



This skeleton, or crustaceous frame-work, consists of a series of 

 rings varying in number, the normal number of the body-segments 

 being twenty-one. Instances of a larger number are rare, and a less 

 number seldom occurs ; one or more rings may be apparently absent, 

 but in such cases they will generally be found consolidated as it were. 

 In the embryo the segments are developed in succession from before 

 backwards ; the posterior rings therefore are generally absent when 

 the number is defective. Each ring is divisible into two arcs, one 

 upper or dorsal, the other lower or ventral. Each arc may present a 

 many as four elementary pieces. Two of these united in the mesial 

 line form the tergum ; the sides of this upper arc are framed of two 

 other portions denominated flanks or epimeral pieces. The lower 

 arc is a counterpart of the upper. Two of the four pieces into which 

 it is divisible constitute the sternum, situated in the mesial line, and 

 are flanked by two episteruums. These two arcs do not cohere at 

 their edges, but a space is left for the insertion of the lateral append- 

 ages or extremities which correspond with them. (Milne-Edwards ; 

 Audouin.) 



The one-and-twenty rings above mentioned are generally divisible 

 into three sections of seven each, and may be considered as corre- 

 sponding with the three regions which zoologists have generally 

 consented to recognise in the bodies of the crustaceans, under the 

 denominations of a head, a thorax, and an abdomen ; but the student 

 should be on his guard against the false impressions which, as M. Milne- 

 Edwards observes, are likely to arise from these terms, by their leading 

 the mind to liken them to the grand divisions in the Vertebrata, which 

 are defined by the same expressions. 



The cephalo-thoracic portion and carapace first claim our attention, 

 and the latter acquires its greatest development in the Decapods. 

 " In these animals," says M. Milne-Edwards, " the frame-work of the 

 body does not appear at first sight to consist of more than two por- 

 tions, the one anterior, formed by the carapace, and representing the 

 cephalic and thoracic segments conjoined ; the other posterior formed 

 by the abdomen. In reality, the first fourteen rings of the body are 

 covered by this enormous buckler, and are so intimately conjoined 

 as to have lost all their mobility ; the whole of the thoracic segments 

 thus hidden below the carapace are connected with it in their superior 

 parts ; they are only joined with one another underneath and late- 

 rally ; and their tergal parts having, in consequence of this, becomo 

 useless, are no longer to be found, being in some sort replaced by the 

 great cephalic buckler ; thus the whole of these rings, in conformity 

 with this arrangement, are imperfect and open above." 



The subjoined cut represents the carapace of a Brachyurous or 

 Short-Tailed Crustacean, and the regions of which it is composed, 

 named after the viscera and organs protected by them. 



The succeeding figure represents the carapace of a Macrourous or 

 Long-Tailed Crustacean. 



The abdomen is most fully developed in the Macroura, or Long-Tailed 

 Crustaceans, in many of which it becomes a very important organ of 

 motion, and in them there is a comparatively small development of 

 the carapace ; while in the Bi-adtyura, or Short-Tailed Crustaceans, 



