CRUSTACEA. 



753 



number, their form, their relations, &c. depend 

 the differences that occur in the conformation 

 of the solid frame-work, the anatomical study 

 of which is now about to engage our atten- 

 tion. 



The most prominent feature in the external 

 skeleton of the Crustacea is common to the 

 whole grand division of articulated animals, and 

 consists in the division of this envelope into a 

 series of segments or rings, connected in suc- 

 cession one with another, and supporting tu- 

 bular appendages, also divided into segments, 

 and arranged endwise. This peculiar structure 

 is met with among the whole of the Crustacea; 

 but when the frame-work of these animals is 

 examined more narrowly, variations are disco- 

 vered so extensive and so numerous, that the 

 mind is almost led to regard it as consisting of 

 elements essentially different. Yet this is not 

 so; and in pursuing the study, aided by the 

 means of investigation developed in the pro- 

 gress of the philosophy of the natural sciences, 

 very opposite results are elicited, results which 

 are replete with interest and instruction in 

 regard to the mysteries of nature in her creative 

 energies. 



Now these methods of investigation may be 

 reduced to two : the first, which studies crea- 

 tures at their full growth, after having ar- 

 ranged them according to the natural order 

 which follows from the investigation of their 

 organization : the second, which studies each 

 creature, but the more perfect in preference, in 

 the series of successive evolutions which 

 constitute the different phases of the em- 

 bryonic state and of extra-uterine life ; for it 

 is a demonstrated fact that these two series, 

 so distinct, so widely separated in appearance, 

 are in reality connected by links so inti- 

 mate, that the one is, in certain respects, the 

 permanent reproduction of the other, which is 

 the continual repetition of the first in one and 

 the same individual. 



By studying in this relative or comparative 

 manner the skeleton of the Crustacea, we suc- 

 ceed in reducing to common principles the 

 mode of conformation, apparently so various, 

 of this apparatus, in the different groups formed 

 by these animals. A remarkable tendency to 

 uniformity of composition is every where re- 

 cognizable, and all the varieties are explicable 

 in a general way by the laws in conformity with 

 which the development of these animals takes 

 place. 



During the period of embryonic life the body 

 is seen becoming divided into rings more and 

 more numerous, and more and more unlike 

 one another. The same tendency to diversity 

 in the organization is also found in the types of 

 which the series of Crustaceans consists; and in 

 both instances the differences are readily seen to 

 depend on various modifications undergone by 

 parts originally similar. It is farther referable 

 to one of the most general laws of organiza- 

 tion, viz. the tendency which nature shows to 

 perfect functions by subdividing the work to be 

 done, and throwing it upon a greater number of 

 special organs. And we observe, in fact, among 

 the most inferior animals that the different seg- 



ments into which the body is divided are so 

 completely repetitions of one another, that 

 they all act precisely in the same manner; 

 they severally include the elements necessary 

 to the display of the vitality distinctive of the 

 entire system to which they belong, so that 

 they may be dissevered without any function 

 whatsoever being therefore the less completely 

 performed in either of the detached portions. 

 Many Annelidans present instances of this 

 uniformity of composition. As we rise, how- 

 ever, in the scale of beings, the different seg- 

 ments of the body are found to become more 

 and more unlike, both as regards their func- 

 tions and their conformation. 



This law is also visibly manifested among the 

 Crustaceans, whether they be studied at the 

 various epochs of their embryonic state or 

 compared together, examples being selected 

 from the different groups of which this portion 

 of the animal series consists. In either case a 

 well-marked tendency to subdivision of the 

 physiological operations is conspicuous; and 

 in proportion as the divers acts, the aggregate 

 of which constitutes the life of the individual, 

 become attached to a particular system or 

 place, the parts to which different functions 

 are apportioned, acquire forms more dissimilar 

 and more appropriate to their peculiar uses. 

 When we come to treat of the evolution of the 

 embryo of the Crustacea, we shall have occa- 

 sion to revert to this subject, but it is neces- 

 sary so far to hint at it in this place, inasmuch 

 as the conclusions which have been mentioned 

 will often supply us with means of explaining 

 those difficulties that are encountered when we 

 seek to render comparative the study of the 

 different constituent parts of the external ske- 

 leton of the articulated series of animals. 



The frame-work or solid parts of the Crus- 

 tacea consist, as we have said, of a series of 

 rings. 



The number of these rings may vary, but 

 this happens to a much less extent than on a 

 superficial view we might be led to conclude. 

 By calling in to our aid the principles of ob- 

 servation and of comparison pointed out above, 

 we have found that in every member of this 

 class of animals the normal number of seg- 

 ments of the body is twenty-one. But 

 a very few instances of a larger number oc- 

 curring are known, and it seldom happens 

 that the number falls short of that which has 

 been indicated. Occasionally, it is true, one 

 or more rings prove abortive, and are never 

 developed ; but in general their apparent ab- 

 sence depends entirely on their intimate union 

 one with another, and other obvious indica- 

 tions of their existence may be discovered. 

 By-and-by we shall find that in the embryo 

 these segments are formed in succession from 

 before backwards, so that, when their evolution 

 is checked, the later rather than the earlier 

 rings are those that are wanting ; and in fact it 

 is generally easy to see in those specimens of 

 full-grown crustaceous animals whose bodies 

 present fewer than twenty segments, that the 

 anomaly depends on the absence of a certain 

 number of the most posterior rings of the body. 



