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spirally twisted arms, and are distinguished by other not less im- 
portant particulars. The soft parts are differently arranged within 
the shell from those of other Bivalves ; the valves are not united by 
any ligament, and there is a very distinct change in the arrangement 
and position of the breathing apparatus. Although Pallas has given 
a short anatomical description of the Terebratule, it was not until 
the appearance of Cuvier’s memoir on the anatomy of Lingula, that 
the true characters of these remarkable animals became known ; it 
was then determined that the Brachiopoda should be set apart in a 
separate and distinct class. The anatomy of the Terebratule and 
Orbicule has since been most elaborately set forth by Prof. Owen in 
the Transactions of this Society, and agrees in all its essential par- 
ticulars with that of the Lingule previously described by Cuvier ; 
subject, however, to certain modifications arising from the different 
situations they inhabit. The Lingule, which are provided with a 
long pedicle, commonly live near the surface, and are found at low 
water, partially buried in the sand for the protection of their fragile 
shells against the violence of the tides ; the Terebratule, on the con- 
trary, are found in deep water, attached in clusters to fragments of 
rocks and corallines by a bunch of short fibrous tendons issuing 
through an orifice in the shell. 
«The essential points in which these animals differ from other 
Bivalve Mollusks are as follow :—First, in the position of the soft 
parts within the shell: in the Brachiopoda the dorsal part of the 
visceral mass is against one valve, and the ventral part against the 
other; whilst in most of the Tropiopoda the back is placed directly 
against the hinge, and the sides against each valve. Secondly, in 
being provided with a pair of retractile brachia or arms: in the 
place usually occupied by the branchiz, are two long spirally twisted 
arms, generally more or less fringed, and so strongly resembling 
in some species the branchize of the Tropiopoda, that they were 
at one time thought to be the true organs of respiration. These 
retractile arms are said to be in constant activity for the purpose of 
producing an inward current of water for the capture of animalcule, 
and other alimentary prey. Thirdly, in the arrangement and posi- 
tion of the branchie: instead of the organs of respiration being di- 
stinctly formed in lateral lamellz upon the body, as in the Lamelli- 
branchiate Tropiopoda, they consist of a number of beautiful veins 
and arteries incorporated within the substance of the two lobes of 
the mantle. ‘The calcifying organ of the Brachiopoda therefore has 
a double function : in addition to its usual property of secreting the 
calcareous mucus for the formation of the shell, it is made subser- 
vient to the circulation of the aérated water. Prof. Owen observes, 
‘that in this profuse distribution of vessels over a plain membra- 
naceous surface, we perceive the simplest construction of the water- 
breathing organ, presenting a beautiful analogy with the elementary 
forms of the air-breathing organ in the pulmoniferous Gasteropoda.’ 
In consequence of this new arrangement of the respiratory system, 
the title of the Brachiopoda has been changed by De Blainville for 
that of the Palliobranchiata, or mantle-breathing Mollusca. The 
