24 THE DEVELOPMENT OF 



mesoderm are coming in between the two halves of the 

 brain, which in the next section (fig. 60) are completely 

 separated by the alimentary canal. In this last section we 

 see two bands of fibres, the outer (n') being the fibrous 

 portion of the nerve going to the anteunula. 



A review of this foregoing shows us that the " brain" at 

 this stage is composed of three pairs of ganglia optic, 

 primitive-cerebral and antennular, the antennal having not 

 yet moved forward beyond the oesophageal collar ; and al- 

 though the two pairs of antennae have acquired a distinct- 

 ively prestomial position, these nervous centres have 

 delayed in the movement. This condition at once recalls 

 the structure in the adult Apus, where Zaddach ('41, pi. 

 in, fig. v) years ago showed that both antennal nerves 

 rise from the oesophageal commissures though Pelseneer 

 ('85) showed that the corresponding ganglia had moved 

 forward to join the primitive brain, although the nerves 

 had tarried behind. 



Of the nervous structures behind the brain at this stage 

 but little is to be said beyond what the figures themselves 

 show. I would call attention, however, to the fact that at 

 this stage the fibrous portions are central and the ganglion 

 cells surround them in the cephalothoracic region, and that 

 this condition persists (fig. 72) in the hatched zoea. In 

 the abdomen (stage //), the fibres are superficial as in the 

 adult (fig. 69). 



In fig. 66, which passes through the sixth pair of appen- 

 dages, may be noticed a couple of patches of pigment, the 

 meaning of which I do not understand. At a little later 

 stage, similar patches appear in pairs in all of the post-oral 

 segments in such a way as to suggest that they may be 

 remnants of segmental sense-organs of the ancestral an- 

 nelid. In surface views, as well as in sections, they ap- 

 pear quite similar to the " nauplius eye," as itappears (figs. 

 17, 18, 20, 54, 56 and 73 oc). My observations on these 



