[72] The rate of increase in dimensions shown by specimens of this species, now so frequently kept in Aquaria, warrants this assertion; though _how many_ years a Crab takes to attain adult size, no exact observations, so far as I know, testify. [73] The exuvia of the cirri are sloughed from the _Balanidae_ about every week in summer; and perhaps this process is coetaneous with an addition to the valves. [74] Mr. Broderip supposes it to have had the power of swimming freely, and of seeking its future habitation, _as a bivalve_; but Loven had not then made known to us the embryogeny and metamorphosis of the _Conchifera_. It is much more probable that the case is as I have ventured to assume in the text. [75] Bennett. [76] Rumphius. [77] The periodical formation of these septa in the progress of growth, is analogous to that of the projecting external plates in the Wendletrap, and of the rows of spines in the _Murex_; but those external processes consist of the opake calcareous layer of the shell, whilst the internal processes in the _Nautilus_ consist of the nacreous layer, like the septa in the _Turritella_. Thus the embryo _Nautilus_ at first inhabits a simple shell, like that of most univalve Mollusca, and manifests, according to the usual law, the general type at the early stage of its existence; although it soon begins, and apparently before having quitted the ovum, to take on the special form.--Prof. Owen's _Lect. on Invertebrate Anim._ p. 593, 2d Ed. [78] Woodward's "Manual of the Mollusca," p. 83. [79] Carpenter, on the Microscope, &c., p. 602. [80] Grant's Comp. Anat., 53. [81] See Jones's General Outline, p. 506. (Ed. 1841.) [82] Such is the common statement. Dr. Harlan, however, observes that "the rattle is cast annually [with the sloughed skin], and, _consequently_, no inference as to the age of the animal can be drawn from the number of pieces which compose the rattles." (_Journ. Acad. Nat. Sci._; v. 368.) I confess this appears to me to be a _non sequitur_; for is it not quite possible that one may be added to the _number_ annually, without involving the actual perpetuity of the preceding ones? It is evident that the increase must take place at some time or other, and it seems to me more likely to occur at the sloughing of the skin, that is, annually, than either oftener or seldomer. [83] Martin "On the Horse," p. 111. [84] Professor Owen's "Odontography:"--to which splendid work I am indebted, for the engravings of these skulls. [85] Brewster's Edinburgh Encyclopaedia. [86] Owen's Odontogr. p. 631. [87] Penny Cyclopaedia; _art._ BONE. [88] Dr. Carpenter's Human Physiol. p. 916. (Ed. 1855.) [89] Sir Thomas Browne, indeed, denies Adam a navel; I presume, however, physiologists will rather take my view. Sir Thomas did not know that the prochronism which he thought absurd pervaded every part of organic structure. The following is his verdict:--