214 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



10, Fig. 50, cl. gn,). Their unclei are nearly spherical, and contain at 

 least one large deeply staining nucleolus. No special preparations were 

 made for the purpose of demonstrating fibrillee in either nerve cells or 

 fibres. Bethe found them in all fibres, and traces of them in the cells of 

 the brain. 



3. Development of the Otocyst. 



For the purpose of comparison with development in the lobster, the 

 antenuules of the first five free swimming larval stages of Carcinus were 

 dissected out, stained and examined in toto. By this means it was 

 ascertained that there is no functional otocyst in the Zoea stages. 



(a) The first Zoea shows no trace of invagination in its antennule. 

 There is, however, an aggregation of nuclei beneath the chitin of the 

 region where the otocyst is to appear. 



(b) The second Zoea shows a slight depression on the dorsal side of 

 the antennule, and its basal portion has begun to widen. 



(c) In the third Zoea this widening has increased, and the lateral wall 

 of the antennule has now formed a rounded protuberance. The invagi- 

 nation has increased in size and depth, but no hairs nor otoliths ax'e yet 

 contained in it. 



(rf) At the Megalops stage we find that a sudden development has 

 taken place, as in the fourth larval stage of the lobster. The Zoea has 

 by a single moult become metamorphosed into a Megalops, and the oto- 

 cyst changed from a shallow depression to a nearly closed sac, contain- 

 ing sensory hairs and otoliths. Two sensory cushions are present : one 

 of these, posterior and median, bears 25 to 30 hooked hairs, upon the 

 tips of which otoliths rest ; the other prominence projects from tlio 

 anterior portion of the median wall, and bears a vertical row of about 30 

 hairs, the shafts of which are directed laterally. These hairs are long, 

 attenuate, and well fringed with delicate filaments. They do not come 

 into contact with the otoliths, and, as already noted, they develop into 

 the thread hairs of the adult ; those of the first sensory cushion described 

 correspond to the hook hairs of the mature crab. The third type of hair 

 found in the adult is not developed at this stage. The aperture is 

 anterior and lateral in position, and extends transversely across the 

 antennule. 



(e) The next stage examined was that of a young crab probabl}' of 

 the stage immediately succeeding the Megalops larva. The otocyst is 

 slightly larger, and its opening is already nearly closed. As a result, only 

 a few small otoliths were contained in it. 



