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brated notochordal column, the “ parachordals ” of Myxine being formed 
of soft cartilage continuous with that of the notochordal sheath (Core 6), 
the primitive condition of the auditory organs, the primitive structure 
of the heart (a simple twist in the persistent sub-intestinal vessel 
provided with muscles), gut (absence of regions), thyroid gland, urinal 
and genital systems (absence of connection between the two, absence 
of genital ducts, &c.—Prick 31), the absence of medullated fibres and 
the structure of the spinal cord (Ransom and THompson 32) and brain 
(“In the lowest Vertebrates, the Cyclostomes, a large part of the 
reticular substance of the brain remains in its primitive indifferent 
condition ; few special nuclei are developed and the activities of the 
animal are developed and the activities of the animal are correspon- 
dingly simple. To any stimulus that may come the animal can respond 
only in a very limited number of ways... In the whole hind-brain 
region no special nuclei in the reticular substance have been found. 
Even the secondary gustatory nuclei are so little developed that they 
have not yet been seen. The cerebellum is “wholly unspecialized,” and 
the fore-brain is the ‘‘most primitive known” (JoHNSToN, 23). 
Part II. Notes descriptive of the Skeleton, some of the Principal Muscles 
and other Features of the Gross Anatomy of the Marsipobranch Genera, 
Geotria and Mordacia. 
Our knowledge of the anatomy of the Petromyzontes -having been 
derived from the almost exclusive study of the commoner species of 
Petromyzon, no apology is needed for the description of any portion 
of the anatomy of other genera of these aberrant Chordata. 
I am indebted for my material to the kindness of Professor 
J. P. Hırı, D. Sc. F.R.S. and Professor A. Denpy, D. Sc., F-RS. The 
greater part of this material, presented to me by Professor Hırr, had 
been preserved in spirit since 1896 and consisted of two adult speci- 
mens and two Velasia stages!) of Geotria australis, all from Tasmania, 
and one adult specimen of Mordacia mordax from Yarra, but I also 
possessed one very well preserved Velasia head of Geotria australis, 
presented by Professor Dexpy, which, mounted as a complete series 
of transverse sections, has been of great service in confirming dissections. 
1) One of these Velasia stages was labelled Geotria chilensis (see text). 
