THE NOTOCHORD AND ALIMENTARY CANAL 443 
of any kind exist; this smoothness is due to the presence of a very 
well-defined cuticular layer secreted by the underlying epithelial cells. 
This cuticle is very much thicker than is usually found in vertebrates, 
and, strangely enough, has been thought to contain chitin. Whether 
it really contains chitin or not I am unable to say, but it certainly 
resembles a chitinous layer in one respect; it is perforated by innu- 
merable very fine tubes or canaliculi, along which, by appropriate 
staining, it is easy to see the secretion of the underlying cell pass 
to the exterior (Fig. 140). This marked digestive power of the skin 
of Ammoccetes, together with the easy passage of the secretion 
through the thin cuticular layer, renders it almost certain that a tube 
formed from the deep ventral groove of the trilobite would, from the 
very first, act as a digestive as well as an absorbent tube; in other 
words, the notochord as soon as formed was able to act asan accessory 
digestive tube. 
This suggested origin of the notochord from a groove along the mid- 
ventral surface of the body not only indicates a starting-point from a 
markedly segmented portion of the body, but also points to its forma- 
tion at a stage previous to the formation of the operculum by the 
fusion of the two foremost mesosomatic appendages—indicates there- 
fore its formation at a stage more nearly allied to the trilobite than to 
the sea-scorpion. The chance of ever finding any direct evidence of 
such a chordate trilobite stage appears to me exceedingly improbable, 
and I greatly fear that this conception of the mode of formation of, 
the notochord can never be put to direct proof, but must always 
remain guesswork. 
On the other hand, evidence of a kind in favour of its origin from 
a segmented part of the body does exist, and that evidence has this 
special value, that it is found only in that most primitive animal, 
Amphioxus. 
This evidence is as follows :— 
At fairly regular intervals, the sheath of the notochord is inter- 
rupted on each side of the mid-dorsal line by a series of holes, which 
penetrate the whole thickness of the sheath. This dorsal part is 
pressed closely against the spinal cord, and through these holes fibres 
appear to pass from the spinal cord to the interior of the notochord. 
So greatly do these fibres present the appearance of ventral roots to 
the notochord, that Miss Platt looks upon them as paired motor roots 
to the notochord, or at all events as once having been such motor 
