salivary glands (b). The brain-ring (e) has on each side a large three-lobed ganglion (f), whence numerous nerves pass to the surrounding parts, and two long branches (h) extend backwards along the sides of the abdomen, to two ganglia (i,i), placed above the muscular foot. 
NERVES OF BULLA Behind these are two sympathetic ganglia, (k, k), which send threads to the digestive system, the ovary (o), the oviduct (p), the uterine sac (q) , the vulva (m), and the urinary organs (n). This may be considered as a fair average sample of the nervous system in the Mollusca, being selected from a Class presenting neither the highest, nor the lowest forms of organization.
The nervous centres are, for the most part, grouped without regard to symmetry, those of one side not corresponding to those of the other; and this irregularity is characteristic of the whole Division, not only in the nerves, but in the other organs of the body. Some zoologists have derived from this peculiarity, a name for the Division, suf-
