670 R. C. PUNNETT. 
gonad spells increased bulk and in the absence of a specialized vascular system this increase 
of bulk must be almost entirely dependent on increased length. For respiration would depend 
on transepidermal diffusion, a process which would more readily occur in a long slender 
animal than in a short thick one. This great elongation of the gonad would necessitate 
the establishment of accessory ducts by which its products could readily and rapidly escape. 
From this it is but a short step to the stage of a series of independent gonads each with 
its own duct extending throughout the elongated trunk region—a condition physiologically 
comparable to that now found among the Nemerteans. Increase of bulk will still mean 
increased fertility and such individuals will be favoured by Genetic Selection’, provided always 
that the means for ensuring due aeration are adequate, whence we come to the establishment 
of dermal pits for the aeration of the gonads. Later the pits become perforated and no 
doubt these perforations supplied a physiological need and filtered off the excess of water 
from the sand passing through the animal’s alimentary canal. Probably it was somewhere 
near this phylogenetic epoch that the ancestral Enteropneusts took to an arenicolous life. 
The problem of extraction of nutriment from a relatively enormous mass of innutritious 
substance resulted in the specialization of the middle and hinder part of the digestive tract, 
and the establishment of the hepatic caeca led to the disappearance of the gonads in this 
region, They have left traces of their former presence in the epidermal annulations which 
shew some tendency to irregularity now that their determining cause has disappeared; they 
are still found in a rudimentary state in the hepatic region of some forms, but increased 
physiological specialization has on the whole led to their confinement to the more anterior 
portions of the trunk. 
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794. Morcaan, T. H. “The Development of Balanoglossus,’ Journ. Morph. 
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1 Cf. Pearson, K., he Grammar of Science, 1900, p. 437, and also T’he Chances of Death, etc., 1897, Vol. 1. p. 63 
(Essay on Reproductive Selection). 
