The ganglia of Malletia are in general plan similar to those of Nucula, the greatest differences being in the cerebral mass. The cerebro-pleural mass seems almost one. In most sections it is very compact and a little more complicated in structure than the ganglion of Nucula. However there are two small ventral ganglionic branches or small ganglia attached to the ventral side of the cerebral mass. These small ganglia may represent the visceral. Farther back in a cross section series as the cerebral mass disappears two other small branches take origin and run parallel to the nerves from the ganglionic cords. These two branches on each side seem to run together before the pedal ganglia are reached. Neither of these pairs of nerves seems connected with an otocyst.
At the cephalic end of the cerebro-pleural ganglion the large ganglionic cords are in evidence. A little distance from the cephalic end on the dorsal side there are quite large groups of cells down from the surface and surrounded by nerve fibers. The course of the fibers here is quite complex. On the ventral lateral sides of the ganglia are paired light areas of fibers which may be traced into the fibers of the ganglionic cords.
The pedal ganglion is small and much as in Nucula. The visceral ganglia are larger and widely separated.
In both Nucula and Malletia young specimens were used for study. In Nucula there was more the appearance of four ganglia in the cerebro-pleural mass, and the ganglia seem less complex than in Malletia. This last species has more separate pleural ganglia, if the ganglionic cords can be so regarded.
In neither of the species studied were all parts of the connectives easy to follow, so it was impossible to test the suggestions of Drew, but in both species there is some indication of two lateral lobes of the cerebral mass, and in Nucula there is good evidence of two central ganglia as well as the smaller lateral ones. The lateral ganglia of the cerebral mass are most clearly separated in Malletia. In Nucula the lateral ganglia are larger in proportion and the distribution of the gray and white matter is more irregular.
REFERENCES
Drew, G. A.1901
The life history of Nucula delphinodonta. Quart, jour. sc. vol. 44, pt. 3.
Pelseneer, P.1891
Contribution á l'étude des Lamellibranchs. Arch. d. biol. xi.
Stempell1899
Zur Anatomie von Solrmya togata. Zool. Jahrb. Bd. xiii.
(Contribution from the Zoological Laboratory of Pomona College)