1885. } i Progressive and Retrogressive. 235 
dicularia).! They differ from the Acrania in the positions of the 
extremities of the alimentary canal. The mouth is on the top of 
the anterior end of the animal, and is supposed by some anato- 
mists to represent an open extremity of the pineal gland of 
other Vertebrata; while the tract represented by this body, the 
third ventricle of the brain, and the pituitary body of the Crani- 
ata, are the remains of the primitive cesophagus of the Urochorda. 
The anus in the adult tunicates is either dorsal, or it opens into 
the body cavity, as in the young larve. In Appendicularia it is 
ventral (Gegenbaur). 
The history of the Tunicata cannot be traced by palzontolo- 
gists as yet, owing to the absence of hard parts in their structure. 
The evidence of embryology has, however, convinced phylogen- 
ists that the ancestors of this class resembled their larva, and 
that they have as a whole undergone a remarkable degeneracy. 
They have passed from an active, free life to a sessile one, and 
have lost the characters which pertain to the life of vertebrates 
generally. ; 
It was to have been anticipated, however, that all of these an- 
cestral Tunicata did not undergo this degenerative metamorphosis, 
for it is to such types that we must look for the ancestors of the 
other Vertebrata, the Acrania and the Craniata. And here palæ- 
ontology steps in and throws new light on the question. I have 
Pointed out briefly, on another page of the NATURALIST, that a 
second order must be added to the Urochorda, viz., the Antiar- 
cha, in which the anus presents the same position as in the 
Acrania, at the posterior end of the body, while an orifice of the 
upper surface represents the mouth of the Tunicata. To this order 
is to be referred the family of the Pterichthyidz, of which the 
typical genus, Pterichthys, is a well-known form of the Devonian 
period. This genus retained its tail, which was the cause, in con- 
nection with the presence of lateral fin-like appendages, of its 
having been supposed to be a fish, by Agassiz, Hugh Miller and . 
others. It is possible that the American Bothriolepis canadensis 
lost its tail, as in the majority of Urochorda. The tunicate which 
approaches nearest to the Antiarcha is the arctic Chelyosoma. 
From the Antiarcha to the Acrania and Craniata, then, the line 
is an ascending one. 
1See Lankester on Degeneration, Nature Series, 1880. 
? This (March) number, 1885, under “Geology and Palzontology.’ 
