6 MANUAL OF THE MOLLUSCA. 
the Brachiopoda, form the sub-class of Molluscoida. In the 
first edition the Tunicata were described in detail, but they are 
omitted in this for reasons stated in the preface. 
Hive of these modifications of the molluscan type of organi- 
sation were known to Linnzeus, who referred the animals of all 
his genera of shell-fish to one or other of them;* but unfortu- 
nately he did not himself adopt the truth which he was the 
first to see; and here, as in his botany, employed an artificial, 
in preference to a natural method. 
The systematic arrangement of natural objects ought not, 
however, to-be guided by convenience, nor “‘ framed merely for 
the purposes of easy remembrance and communication.” The 
Fig. 7. A Bivalve.+ 
true method must be suggested by the objects themselves, by 
their qualities and relations ;—it may not be easy to learn,—it 
may require perpetual modification and adjustment,—but. inas- 
much as it represents the existing state of knowledge it will aid 
* The Linnzean types were—Sepia, Limax, Clio, Anomia, Ascidia. TZerebratula 
was inciuded with Anomia, its organisation being unknown. 
+ Mya truncata, L.3. From Forbes and Hanley. 
t Ascidia mentula, Mill. Ideal representation; from 2 specimen dredged by Mr 
Bowerbank, off Tenby. 
