934 ^^^ American Naturalist. [October, 
optic thalami, and the hind-brain of vertebrates. In the spinal cord 
we see another parallelism, for this structure in vertebrates closely re- 
sembles the ventral chain of ganglia in crustaceans. 
One difficulty stood in the way of further work on the homology 
between the nervous system of vertebrates and the arthropod nervous 
system, i.e., the question of the invertebrate alimentary canal. Gas- 
kell's theory that this canal is still existent in the vertebrate nervous 
system removes the difficulty. The remains of the old alimentary 
canal are found in the non-nervous tube which is mixed up with the 
nervous system proper. " This alimentary canal was of the type of 
the crustacean canal, a large cephalic stomach, and a straight simple 
intestine opening by means of an anus. The remains of the non- 
nervous cephalic stomach are well seen in the cephalic region of the 
nervous system in the shape of non-nervous epithelial structures, which 
are so freely found here as part of the walls of the central tube, and 
which by being thrown into folds form on the dorsal side the choroid 
plexus, on the ventral side the saccus vasculosus. Remains of the 
mouth and oesophagus are found as a folded down tube forming the in- 
fundibulum with the lobi infundibuli. In this way an explanation is 
given of non-nervous structures found in connection with the nervous 
tube of vertebrates." Now, if Gaskell's theory be true, we ought to 
find in connection with the canal of the vertebrate nervous system in 
the cranial region, some trace of the so-called liver of crustaceans, a 
large and important organ opening by a duct into the pyloric end of 
the cephalic stomach. In man nervous material entirely fills the cran- 
ial cavity, while in the fish the brain lies in a case, the greater part of 
which is filled by a jelly-like material. Gaskell does not consider this 
jelly a packing material, but the remains of some pre-existing organ, 
and this organ he thinks to be the so-called liver. In the study of 
Ammoccetes he finds this material to be arranged in a definite mass 
composed of glandular looking cells, and in a hilus formed in the 
mass he finds the remains of a tube passing from the commencement 
of the fourth ventricle to the surface. The spot where it leaves the 
fourth ventricle is in the region of the ganglion interpeduncidare, which 
according to Ahlborn contains within itself the remains of a diverticu- 
lum from the central cavity of the nervous system. The duct opens 
into the central cavity at the posterior limit of the fourth ventricle,— 
i.e., into the pyloric end of the cephalic stomach. 
Gaskell discusses several structures whose functions are unknown. 
The interpretation of the hypophysis is so closely bound up with the 
question as to whether it is a paired organ or not, that he disregards it. 
