A STUDY IN MORPHOLOGY. 73 



(A), the second antennas (An], and the mandibles (J/). They are not divided into 

 joints or rings, although the second antennae and the mandibles are bi ram cms, and 

 consist of a basal portion or protopodite, an expedite, 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 maxillae 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 (Oc) 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 ('), 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 



