A STUDY IN MORPHOLOGY. 
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(A), the second antennae (An), and the mandibles (M). They are not divided into 
joints or rings, although the second antennae and the mandibles are biramons, and 
consist of a basal portion or protopodite, an expodite, and an endopodite. All three 
pairs have hairs projecting from their tips, and these lengthen considerably within a 
few minutes after the embryo is freed from the egg. The first antennae are nearly as 
long as the second, and both pairs, as well as the mandibles, are organs of locomotion, 
to row the animal through the water. The motions of the larva are very erratic and 
violent, and consist of a series of quick leaps produced by vigorous backward strokes 
of the appendages. 
The outline of the body will be understood by a reference to the figures. When 
the second maxilke are in the centre of the field of view, as in fig. 21, the outline is 
pear-shaped, with the broad end of the pear at the posterior end of the body; but 
when the metastoma is in the centre this is reversed, and the broad end is in front. 
This difference is due to the fact that the dorsal region is much wider than the labrum 
and series of buds, which together form a ridge along the ventral surface. 
In a dorsal view the simple eye (06) is seen as a black spot on the middle line, near 
the anterior end of the body. It did not show any traces of a division into halves at 
any stage of development which was observed. 
The ocellus lies upon a large rounded granular body, which is imperfectly divided 
into halves by a notch upon its posterior margin. This body consists of the fused 
cerebral ganglia. 
The dorsal portion of the posterior region of the body is swollen and rounded, as 
shown in figs. 21 and 23 ; and near its lateral margins there are a pair of small, but 
very conspicuous, dark pigment-spots (i), which might easily be mistaken at this stage 
for ocelli, since they have almost exactly the same size and colour. These two pigment- 
spots are very conspicuous during all the early stages of the metamorphosis, and their 
position during the later stages (figs. 25, 26, 27, 34, 35, and 47, p) shows that the 
portion of the Nauplius body which bears them becomes the thoracic, not the abdominal, 
region of the adult. 
In the interior of the enlarged posterior portion of the body there is a huge mass of 
polygonal highly-refractive bodies, which appear to constitute a food-yolk, and which 
surround the digestive tract of the embryo. I have already given my reasons for 
believing that those bodies are derived from the spherule which becomes pushed into 
the segmentation cavity during the early stages of development. If this is their 
origin they must increase in size between the stage shown in fig. 20 and that shown 
in fig. 21. This is not at all an unusual occurrence, and in the fresh- water Pulmonates 
the yolk-spherules which surround the digestive tract continue to grow until a very 
advanced stage of development. I found so few eggs at this stage that I was afraid 
to sacrifice any of them by attempting to study their internal structure under pressure, 
and I am not able to give an account of the digestive tract or of the other internal 
organs. 
MDCCCLXXXII. L 
