702 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the space around the brain because it represents some pre-existing organ 
which was of importance to the animal from which the Vertebrate 
sprung ; such an organ is clearly the “ liver ” of Crustacea. The signi- 
ficance of the pigment is next discussed. 
From his study of the optic organs, the author concludes that the 
original Crustacean-like ancestor had a pair of median eyes, each with 
its optic ganglia and connections with both supra- and infra-oesophageal 
ganglia ; the right eye remained functional longer than the left. 
With regard to the structure of the supra- and infra-oesophageal 
ganglia, Dr. Gaskell points to the fact that both the Crayfish and the 
Ammocoetes have giant-cells, large cells, and small cells. It is because 
the central nervous system of the Vertebrate is the direct descendant of 
the Arthropod that there is such similarity between them, and not be- 
cause, as Bellonci supposes, similarity of function requires similarity of 
structure. The author just quoted has directed particular attention to 
the close similarity of the olfactory organ of the two groups. In the 
Ammocoetes the olfactory glomeruli resemble exactly in appearance the 
reticulated substance (“ Punctsubstanz ”) of the Arthropod nervous 
system. But the author postpones the discussion on this point to the 
next chapter, in which the cranial nerves will be examined. 
Origin of Vertebrates from Arachnids.* — Dr. W. Patten was first 
led to suspect that the Arachnida were the ancestors of Vertebrates from 
the fact that concentration and specialization of head-segments is, among 
Invertebrates, greatest in the Arachnida. The points which he now 
attempts to prove are (1) that in the Scorpion the cephalothoracic 
neuromeres, sense-organs, and mesoblastic somites present, in a general 
way, not only the same specialization and the same numerical arrange- 
ment in groups, but also the same difference as a whole from the body- 
segments, as do the corresponding parts in the Vertebrate head ; (2) that 
the Arachnid cartilaginous sternum represents the primordial cranium 
of Vertebrates ; (3) that in the Trilobites and Merostomata the internal 
structure of the cephalothorax resembles in some respects that of Scorpio 
and Limulus ; (4) that the remarkable fish-like Pterichthys and related 
forms, judging from their external structure, are closely related to the 
Merostomata, and serve to connect Arthropods with Vertebrates ; and 
(5) that the embryology of Vertebrates in its main features can be 
reduced to the Arthropod type. 
The author brings forward suggestions rather than facts in support 
of his theory, but his views as to the classification of animals may be of 
some interest. (See Table, p. 703.) 
New Theory of Pterichthys. f — Mr. A. Smith Woodward subjects 
Dr. Patten’s speculations as to fossil fishes to a severe criticism. 
He says that when it is suggested that the so-called dorsal shield 
of Pterichthys is on the haemal aspect of the animal, an ichthyologist, 
at any rate, is unable to regard the statement as anything beyond 
unjustifiable speculation. The figures copied by Dr. Patten have been 
shown by Traquair to be inaccurate ; the (i cervical suture ” of Pterichthys 
is nothing more than a superficial slime-canal, and the same remark 
* Quart. Journ. Micr. Sci., xxxi. (1890) pp. 317-78 (2 pis.). 
f Ann. and Mag. Nat. Hist., vi. (1890) pp. 314-6. 
