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E. W. MACBRIDE. 
broad, and can only maintain its equilibrium so long as it 
moves. When it ceases moving it falls on one side, and this 
is true of the larvm of Amphioxus, as Willey has pointed 
out (37). Between the condition of life when the ancestors 
of Amphioxns were entirely pelagic and the condition in 
which they burrowed, thei’e intervened in all pi’obability a 
transitional condition when they lay on their left 
side on the bottom, making only occasional excursions 
through the water. In this “ Pleuronectid stage,” as we 
may call it, which in all probability represents the mode of 
life of the older Am ph ioxu s larvae, it would be advantageous 
to twist the originally mid-ventral mouth on to the left side 
so as to improve its opportunities of feeding, and at the same 
time to twist the left gill-slits on to the upper right side so as 
to remove them from the substratum and place them in a 
favourable position for evacuating water. 
The twist of the mouth seems to have been effected by a 
greater growth of the right side of the animal, and hence it 
comes about that the endostyle is at first anterior to the 
mouth, and so is also the opening of the club-shaped gland 
(fig. 17 })). But in the region of the gill-slits the opposite 
kind of an inequality of gTowth must have subsisted, for by 
greater growth of the left side the left gill-slits are pushed 
on to the right side, and the mid-ventral line is displaced 
high up on the right side. Very similar conditions prevail 
in the head of a developing sole, for in this animal the mouth 
is twisted downwards and the left eye upwards. In the 
ontogeny of the individual, however, these inequalities in 
growth have been hurried on before their time, and the 
reason why the left gill-slits only (Willey’s primary gill-slits) 
are found in young larvm is that in larvte of small size, when 
there ought to be a number of similar structures performing 
the same office. Nature reduces the number owing to con- 
siderations of want of space (cf. the Nauplius larva of 
Crustacea with only three pairs of legs, and the single lateral 
cerebral eye in the Ascidian tadpole). 
Van Wijhe (35) has given a somewhat similar explanation. 
