OS SOME ARMOURED FISHES. 
3 
There is no swim-bladder, no special heart, the circulation 
being effected by means of several contractile dilatations — 
44 pulsatile portal sinuses ” (Owen) — developed upon the main 
blood-vessels. The blood is colourless. A pair of nerves are 
given off to the eyes (op), and another branch to a ciliated pit, 
probably an olfactory organ (ol). 
The resemblance between the respiratory pharyngeal sac of 
the 44 lancelet ” and that of the tunicated mollusks, Ascidians, or 
44 sea-squirts,” was pointed out by the late Professor Groodsir,* who 
also considered the 44 Lancelet” as allied to the Annulosa, from the 
simple organization of its respiratory and circulatory system. 
In the more recent researches of M. Kowalevsky, he traces 
a close affinity between Amphioxus and the early stages of 
some Ascidians. 
44 Thus in Amphioxus are united characters belonging to 
the Tunicata and Annelida, and unexpected relations are re- 
vealed between the Vertebrata and Invertebrata.” f 
Yet even if we might consider Amphioxus to be, like Am- 
mococtes , an immature form, which perhaps reached a somewhat 
higher adult stage by a tardy metamorphosis,! there are many 
other animals besides the 44 Lancelet ” in which the skeleton in 
the adult retains many of its embryonic characters, and although 
these are restricted at the present day to a few among the 
living orders of fishes, such characters were common to all 
the older fossil forms. 
Professor Owen observes 44 the centrum is the most constant 
vertebral element as to its existence, hut not as to its ossifica- 
tion. There are some living fishes, and formerly there were 
many, now extinct, in which the peripheral elements of the 
vertebra become ossified, the central one remains unossified. ”§ 
This central basis of the neuro-skeleton, as we have already 
seen in the case of Amphioxus , may attain in the adult animal 
to no higher condition than that in which it is laid down in the 
embryo of every vertebrate animal, namely, as a more or less 
cylindrical fibrous sheath, filled with simple cells containing 
jelly. In the Lamprey (. Petromyzon marinus ) cartilaginous 
processes strengthen the sides of the neural canal. In the 
Sturgeon ( A cipenser sturio) the inner layer of the notochord 
has assumed the texture of tough hyaline cartilage, whilst 
* 44 Trans. Roy. Soe. Edinb.” Yol. xv. p. 259, Plate xi. 
t 44 On Certain Genera of Living Fishes and their Fossil Affinities. ,, By 
Miss Agnes Crane. 44 Geol. Mag.” 1877, Vol. iv. p. 209. 
t Ammoccetes, after the third year from the egg, passes by metamorphosis 
to Petromyzon. (Owen, 44 Comp. Anat. Verteb.” Vol. i. 1866, p. 7.) Save that 
Ammocoetes inhabits mudbanKs in rivers, whilst Amphto.ms burrows in sand 
beneath the sea, these forms agree closely with each other. 
§ Owen, 44 Comp. Anat. Verteb.” Vol. i. 1856, p. 30. 
