MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 159 
shall compare them in the first place with the Lamellibranchiate Bivalves, to which they 
present the most obvious relations in the nature and forms of their organs of defence. 
To these they are in some respects superior. The labial arms are more complex pre- 
hensile organs than the corresponding vascular /amine on either side the mouth of the 
Lamellibranchiata. The whole muscular system is more complex; and the opening 
as well as the closing of the shell being regulated by muscular action, indicates a higher 
degree of organization than where the antagonizing power results from a property of 
the cardinal ligament, which is independent of vitality, viz. elasticity. With respect, 
however, to the respiratory organs, the modifications which these have presented in 
Orbicula and Terebratula show the Brachiopods to be still more inferior to the Lamelli- 
branchiata than was to be inferred from the structure of the branchie in Lingula: and 
notwithstanding the division of the systemic heart, I consider that there is also an 
inferiority in the vascular system. Each heart, for example, in the Brachiopoda is as 
simple as in Ascidia, consisting of a single elongated cavity, and not composed of a 
distinct auricle and ventricle, as in the ordinary Bivalves: for in these even when, as 
in the genus Arca, the ventricles are double, the auricles are also distinctly two in 
number ; and in the other genera, where the ventricle is single, it is mostly supplied 
by a double auricle. The two hearts of the Brachiopoda, which in structure resemble 
the two auricles in the above Bivalves, form therefore a complexity or superiority of 
organization more apparent than real. 
Having been thus led to consider the circulating as well as respiratory systems as 
constructed on an inferior plan to that which pervades the same important systems in 
the Lamellibranchiate Bivalves, I infer that the position of the Brachiopoda in the natural 
system is inferior to that order of Acephala. 
Among the relations of the Brachiopoda to the Tunicated Acephala, and more especially 
to the Ascidie, we may first notice an almost similar position of the extended respiratory 
membranes in relation to the mouth, so that the currents containing the nutrient mo- 
lecules must first traverse the vascular surface of that membrane before reaching the 
mouth; the simple condition also to which the branchie@ are reduced in Orbicula and 
Terebratula indicates their close affinity to the Ascidie. But in consequence of the form 
of the respiratory membranes in the Brachiopoda, which is so opposite to that of the 
sacciform branchie of the Ascidie, the digestive system derives no assistance from that 
part as a receptacle for the food, and the superaddition of prehensile organs about the 
mouth became a necessary consequence. The Brachiopods again are stationary, like the 
Ascidie, and resemble the Boltenie in the pedunculated mode of their attachment to 
foreign bodies. 
With the Cirripeds their relation is one of very remote analogy; their generative, 
nervous, and respiratory systems being constructed on a different type, and their brachia 
manifesting no trace of the articulate structure. In all essential points the Brachiopoda 
closely correspond with the Acephalous Mollusca, and I consider them as being in- 
