Frce-'swimming Nauplius of Leptodora liyalina (Lillj.). 213 



much less advanced in the development of the internal organs. The 

 subject of variation in time, and the partial independence of the 

 different organs in development, would seem to be well worthy of 

 more attention than has been paid to it. 



The lower lip appears late ; it seems to originate from paired rudi- 

 ments ; but the slight papillae representing the maxillse do not enter 

 into its formation, for they flatten out and disappear. 



The characteristic shape of the adult thorax, whereby the ventral 

 surface bearing the legs comes to be situated nearly at a right angle to 

 the head, is not assumed, as we might have expected, until the adult 

 Structure is attained. 



Even in the quite young nauplius the ectoderm over the head is 

 curiously modified; the cells are large and possibly glandular or 

 excretory in nature. They possess large nuclei towards their bases 

 -and are much taller than the ordinary ectoderm cells. In the adult 

 animal, these cells form a large patch over the head, the "Kopfschild" of 

 Weismann (fig. 2). I have not detected anything else of the nature 

 -of a " dorsal organ," and I suggest that the above-described structure 

 represents it. 



As the youngest nauplius captured was a free-swimming creature 

 with many muscles, it might have been anticipated that anything of 

 the nature of segmental coelom pouches, if present, would be much 

 obliterated. Most of the mesoderm consists of a fairly uniform sheet 

 of cells lying on each side of the gut. Posteriorly the mesoderm is 

 more abundant and compact. The muscles of the thoracic legs are 

 formed from the base of the mesoderm bands (fig. 3, B). The cells 



B. ' A. 



Pig. 3. — A. Cross- section of a young nauplius just behind tlie rudiment of 2nd 

 maxilla. The exit-duct of the maxillary gland can be seen passing up into 

 the dorso-lateral swelling. 



B. Cross-section of a slightly older nauplius ; it is a little posterior to A. 

 Differentiation of end-sac and part of glandular tube can be seen in the dorso- 

 lateral swelling. 



