37 



brancliigerous at any period of tlieTrilobite's existence ; but, if tbe ridged (wbat a car- 

 penter would call ' beaded ') inferior borders of the eight thoracetral segments have not 

 been so misinterpreted *, are slender, filamentary, cylindrical, jointed ambulatory limbs, 

 terminated by a claw f. tinder either alternative the difference is great as compared 

 with the coalesced pairs of broad lamelliform articulate appendages of the thoracetron of 

 the larval (fig. 8) as of the matm-e Limulus (Pis. II. & III.), and still greater when the 

 Trilobitic larva, with its pleou or pygidium for the second body-part, (fig. 12, a, b), is 

 compared with Avhat is termed the " Trilobitenstadium " of the Limtdus ; in which 

 stage one sees, with the thoracetron for the second body-segment, beneath it already 

 developed three or more of the lamelliform limbs, on the second and third of which the 

 gill-plates have begun to appear J. This is far from being a 'Trilobite;' and nothing 

 is gained to science by putting figurative expressions for facts. In the inductive school 

 of biology, the notion that a higher form traversed a series of lower forms in the course 

 of its development has ceased to be set forth, save under duly modified terms §. I am 

 under the impression (and it is an agreeable one to the mind searching solely for intel- 

 ligible and demonstrable conclusions) that few now dispute the fact that each individual 

 of a given species is such ab initio, and takes its own course to the fuU manifestation of 

 its specific characters, agreeably with the nature originally impressed upon the germ. 

 A King-crab does not, any more than a perch, a dog, or a man, begin to be such only 

 when the zoologist discerns the respective characters of the parent, but is such even 

 before embryologists detect their earliest dawn. The embryo Limulus derived its nature 

 and the potency of growth according to the specific pattern from the moment of tlie 

 impregnation ; and each step of development moves to the consummation of the pattern 

 as its end and aim I). The generic character is indeed significantly soon shown in the 

 budding Limulus. 



The first steps, like those in all segmental (whether articulate or vertebrate) animals, 

 recall the work of crystallization, and illustrate growth by repeated samenesses. These 

 show the results of formifaction, aggregated in series of similar heaps of organic atoms 

 (fig. 4) before the specific affinities begin formally to operate thereon and plainly to 

 show themselves to the eye. No sooner, however, can one of these heaps, or pairs of 

 heaps, be recognized as budding limbs, than in such series the first is seen to be Limuline 

 by its halting growth (fig. 6, ii) ; the second (iii) pushes on outside these, the basal 

 joints of the ' antennules ' (ii) being at the interspace of those of the ' antennae ' (fig. 8, 

 III), according to the King-crab's pattern. 



Further back in that interspace opens the mouth (fig. 6). It is at no developmental 

 stage typical as a transient manifestation of the ordinary position of the mouth in an 

 annulose animal; that is to say, it is at no time terminal — but as soon as it opens 

 (fig. 4), testifies by its inferior position that it is the mouth of a Limulus, not of any other 

 or any lower form. 



* I offer this alternative with diffidence, as I have not had the opportunity of examining the exceptional specimen. 



t According to H. Woodward's restoration, in ' Geological Magazine,' July, 1871, pi. viii. fig. 1 a. 



X Packard, he. cit. p. 170, pi. v. fig. 26. 



§ See the concluding Lecture of my course on " Invertebrata," of 1843, 8vo, p. 367. 



II Anat. of Vertebrates, vol. i. p. xxi. 



