468 On Progressive Development {Sepr. 
animal increased ; the ductus arteriosus is obliterated ; its pulmonic 
branches alone give a passage to the blood, the whole of which, now 
undergoing the necessary changes in the lungs, is sent from the sys- 
temic side of the heart to perform its functions in the animal system. ° 
There is now therefore a heart of four cavities, and a perfect system 
of respiration, in short, that of the highest type, birds and mammalia. 
The first appearance of that form of organization which runs through all 
the vertebrated classes is to be found in the most perfectly developed 
tribe of the invertebrata, the naked cephalopoda. The chambered 
and convoluted shell of the nautilus and the ammonite may be traced 
in the internal skeleton of the sepia, which consists of numerous con- 
centric lamelle of carbonate of lime, connected by an infinite number 
of siphonculi running right angles to them. Now suppose each 
lamella separated from that next to it, and the number of connecting 
siphonculi reduced to one between each lamina, and a polythalamous 
shell will be produced. 
Still higher we find in the Joligo a single cartilaginous plate, 
somewhat concave anteriorly, as though its edges were approximating 
to form a tube, enclosed within the mantle, and lying posterior to all 
the organs of respiration, circulation, digestion, &c. This cartilagin- 
ous plate performs the office, though imperfectly, of a vertebral column, 
forming an organ of protection for the nervous system. The carbonate 
of lime, so universal in the external skeletons of all the Mollusca, has 
here entirely disappeared, as though preparatory to the introduction 
of a new element characteristic of the skeletons of the higher classes, 
the phosphate of lime. By a very easy transition from this simple 
skeleton of the Joligo we pass to the lowest of the cartilaginous fishes, 
where in the petromyzon, the vertebral column presents a form almost as 
rudimentory. 
The respiratory and circulatory apparatus in the loligo are very 
nearly the same as in fish, being entirely aquatic; the aeration of the 
blood takes place in the branchiz, placed on each side, hanging freely 
in the cavity of the mantle, and fixed on their dorsal aspect to cartila- 
ginous laminz, which may be considered the rudiments of branchial 
arches. 
The blood brought by the vene cave to two muscular cavities 
called auricles, and thence sent to the branchiz, is returned to a third 
muscular heart, to which the name of ventricle has been given. There 
is here no essential difference from the circulatory organs in fishes, 
but a lower degree of development is indicated in the permanent dis- 
union of the muscular hearts, a concentration of organs being one of 
the most characteristic features in perfection of development. 
