THE ANATOMY OF THE RI VER-M U SSEL. 
253 
Tunicata, e. g. the “ sea-squirts ” (see Popular Science Review , 
July 1869), while the mantle answers to the muscular “inner 
tunic” (Hancock) of the latter, the dorsal junction of its 
lateral lobes corresponding with the line of the Ascidian 
“endostyle.” Professor Allman, moreover, considers that two 
fleshy plates, which at an early stage in the life of certain 
Polyzoa envelope the embryo, answer to the right and left 
mantle-lobes of a Lamellibranch ; an indication of a rela- 
tionship rather with these molluscs than with the molluscoid 
“lamp-shells” ( Brachiopoda ). In further support of this affi- 
nity, it is stated that the labial tentacles of the river-mussel, 
which some consider the homologues of the “ arms” of a 
Brachiopod, answer to the “lophophore” of the Polyzoa. The 
foot of Anodonta is probably a modified representative of the 
“ metapodium,” or most posterior of the three divisions of the 
foot in the snails and slugs ( Gasteropoda ). The branchial 
chamber of the river-mussel according to some, according to 
others, the pharynx, is homologous with the ciliated gill-sac of 
the Ascidian. The glandular portion of the organ of Bojanus 
was considered by Van der Hoeven to be the homologue of the 
appendages of the branchial veins in the Cephalopoda , e. g. the 
cuttle-fish. The labial ganglia of the river-mussel, with their 
commissural cords, answer to the supra-oesophageal nerve mass 
in the Odontophora , e.g. the snail ; while the parieto-splanch- 
nic ganglion is the homologue of the single nerve centre of 
the Tunicata. 
As Mr. Herbert Spencer has well pointed out in his Prm- 
dples of Biology , among the Lamellibranchs “ we have diverse 
forms accompanying diverse modes of life.” Those which fre- 
quently move about, as the river-mussel, and those, such as the 
sea-mussel, which, though fixed, have the two shell-valves 
similarly conditioned, retain the bilateral symmetry charac- 
teristic of the order. In those, on the other hand, such as the 
oyster, where one valve is always undermost, or attached, and 
the other is uppermost, or most under the influence of its 
“environment” — the water — we find, as we ought to expect, a 
want of symmetry. There is further an absence of definition, 
or rather a great variation, of outline in the contained animals, 
which may be accounted for by the mutual interference to 
which they are subject from their clustering together ; the 
“struggle for existence” being carried on in a very limited 
area, and one from which those worsted in the combat are 
unable, leaving their more successful rivals in possession of the 
field, to stir abroad in search for “ pastures new.” 
