1891.] Recent Studies of the Vertebrate Head. 337 
ancestor. The functions of the supracesophageal and the infra- 
sceophageal ganglia and the ventral chain correspond to the func- 
tions of those parts of the vertebrate central nervous system 
which are situated in the same anatomical position, with respect 
to the non-nervous tube, as the corresponding ganglia of the _ 
Crustacean with respect to the alimentary canal. The Crustacean 
cephalic stomach is represented in the brain of the Ammoccetes 
by the choroid plexuses, the continuation of the tissue of the 
latter that lines the cavities of the brain being the ventral portion 
of the stomach walls. The nervous masses lying outside this 
lining epithelium are probably composed of tissue arranged in the 
same way and of the same structure as the supracesophageal, 
infracesophageal, and thoracic ganglia of the Crustacean-like 
ancestor. The two nervous masses which form the brain proper 
and olfactory lobes are in the position of the supracesophageal 
ganglia with respect to the walls of the cephalic stomach, and in 
connection with a special optic portion which gives rise to eyes 
of a strictly Arthropod type. Rudiments of the old mouth and 
cesophagus are seen in the infundibular process. A bilaterally 
arranged mass of pigmented tissue that fills up a large portion of 
the space around the brain is looked upon as the rudiment of 
the Crustacean liver, while its duct is seen in the conus post- 
commissuralis. The pigment is regarded as the remains of the 
blood channels of the old cephalic liver. The original Crustacean- 
like ancestor had a pair of median eyes, represented in the 
Ammoceetes by the “ dorsal ” and “ ventral ” pineal eyes, the dor- 
sal eye remaining functional much longer than the ventral. The 
central nervous system of the Ammoceetes, and therefore of all 
other vertebrates, is the direct descendant of the Arthropodnervous 
system in all respects. The vertebrate alimentary canal is formed 
by the prolongation of a respiratory chamber, the latter contain- 
ing the gill-bearing legs of the ancestral form; the legs being 
still present in Ammoccetes in the form of branchial bars. The 
Segmental cranial nerves are the nerves arising from the infra- 
cesophageal and thoracic ganglia. The first'two cranial nerves are 
_ the nerves of special „sense arising from‘ the supracesophageal 
ganglia.‘ 
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