. The causes and course of organic evolution; . and well-developed cerebral ganglia from whichtwo strong ventral, two rather strong lateral, and two weakerpaired dorsal nerve-threads are given off. But we wouldregard the last of these as of extreme importance. For bygradual approximation and ultimate fusion in higher rhab-doccels and in nemerteans, as their function in relation tothe eye on the one hand and the anterior external muscleson the other became increasingly accentuated, they furnish Evolution of Animals 433 the natural and evolutionarily continuous origin for Reissnersfiber that is
. The causes and course of organic evolution; . and well-developed cerebral ganglia from whichtwo strong ventral, two rather strong lateral, and two weakerpaired dorsal nerve-threads are given off. But we wouldregard the last of these as of extreme importance. For bygradual approximation and ultimate fusion in higher rhab-doccels and in nemerteans, as their function in relation tothe eye on the one hand and the anterior external muscleson the other became increasingly accentuated, they furnish Evolution of Animals 433 the natural and evolutionarily continuous origin for Reissnersfiber that is present in vertebrates generally. In practically all cases the nemerteans show a relativelylarge dorsal brain that consists of paired lobes. Each of theselies directly above or somewhat behind the mouth^in formswhere the proboscis opens in front of and separately from themouth. But in some land nemerteans the main mass of thebrain lies in front of the common orifice of the oesophagusand proboscis-sheath. The dorsal lobes also, as in the verte-. FiG. 17.—a. Nervous system, to left, of Drepanophorus (after Hubrecht). Cdorsal brain; s, anterior sensory nerves; t, nerves to proboscis; ce nerves to digestivetract; L, probable auditory organ and orifice o; T, lateral nerve trunks with theirbranches. 6. Nervous system to right of Hoplonemertean (after Mclntosli).c, dorsal brain; s, probable oKactory and optic nerves; dc, dorsal commissure withjunction of dorsal nerve dn; vc, ventral commissure passing beneath proboscissheath ps; v, ventral ganglion; T, lateral nerve trunks. brates, may in higher types undergo subdivision into an anteriorlobe or thalamencephalon, and a posterior or mesencephalon asin Ewpolia. The former are not only anterior to, they largelyoverlap, the posterior lobes (Fig. 18 a). Anteriorly in somegenera of nemerteans, as in vertebrates, these lobes give offbranches that might appropriately be termed in the formergroup, did we know their function exactly, the olfac
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