144 THE PLACOID BRAIN. 



with its provision-knapsack slung fast to it, it reminds one 

 disposed to be fanciful, save that its burden is on the wroni^ 

 side, of Scottish soldiers of the olden time summoned to attend 

 their king in war, — 



*' Each on his back, a slender store. 



His forty days' provision bore, 



As ancient statutes tell." 



Around that terminal part of the creature's body traversed 

 by the caudal portion of the vertebral column, which com- 

 mences in the salmon immediately behind the ventrals, there 

 runs at this period, and for the ensuing five weeks in which 

 it does not feed, a membranous fringe or fin, which exactly 

 resembles that of the tadpole, and which, existing simply- as 

 an expansion of the skin, exhibits no marks or rays. In the 

 place of the true caudal fin, however, we may detect, with 

 the assistance of a lens, an internal framework with two well- 

 marked lobes, and ascertain, further, that this tail is set on 

 awry, — the effect of a slight upward bend in the creature's 

 body. And when viewed in a strong light as a transparency, 

 we perceive that the spinal cord takes the same upward bend, 

 and, as in the sturgeon, passes in an exceedingly attenuated 

 form into the upper lobe. What may be regarded as the de- 

 sign of the arrangement is probably to be found in the pecu- 

 liar form given to the little creature by the protuberant bag 

 in front. A wise instinct teaches it, from the moment of its 

 exclusion from the egg, to avoid its enemies. In the instant 

 the human shadow falls upon its pool, we see it darting, with 

 singular alacrity, into some recess at the sides or bottom ; and 

 in order to enable it to do so, and to steer itself aright, — as, 

 like an ill-trimmed vessel, deep in the water a-head, the ba- 

 lance of its body is imperfect, — ^there is, if I may so express 

 myself, a heterocercal peculiarity of helm required. It has 

 got an irregularly-developed tail to balance an irregularly- 

 developed body, as skiffs lean on the one beam and full op. 



