128 BRITISH PALAZOZOIC FOSSILS. [ANnuLaTA. 
is driven from the anterior part of the dorsal vessel by a series of vertical, moniliform, muscular hearts, 
the current rushing towards the tail, where it enters the great dorsal vessel by anastomosing branches, and 
helps to propel its contents towards the head. Most of them are hermaphrodites. The eyes, when 
present, are formed of a simple expansion of the optic nerve, covered by a transparent cornea, without 
any of the usual lenses or humours. 
The class Annulata is divisible into three orders: Ist, Abranchia; 2nd, Dorsibranchiata; 3rd, 
Tubicola. 
ist Ord. ABRANCHTIA. 
This group is distinguished by the absence of external branchi, respiration being effected partly by 
the surface of the skin, and partly by internal vascular sacculi opening by external spiracles. It contains, 
as tribes, first, the Annulata Suctoria or leeches; second, the Annulata Terricola or earth-worms; and 
third, the little-known Nemertina, a marine group of very long, slender, smooth worms, without antenna, feet, 
or other appendages to the skin, and very indistinctly annulated. ‘To this group has been provisionally 
referred the Silurian genus, Nemertites. 
Genus. NEMERTITES (Mace Leay). 
Gen. Char.—Body very long, linear, slender, of nearly uniform thickness throughout, without distinct 
articulations. 
Only one species of this supposed genus of Nemertina has occurred, and it is very doubtful whether 
it and the so-called Gordiw of the American Taconic slates may not belong more properly to the vege- 
table kingdom. (See the genus Palwochorda, M*Coy, Pl. 1. A.) 
NEMERTITES OLIVANTI (A/urch.) 
Ref— Murchison Sil. Syst. t. 27. f. 4. 
This body occurs usually in long loop-like folds, about half an inch apart, and three or four inches 
long. The diameter of the smooth subcompressed body is rather more than half a line; the length many 
inches, but unknown. 
Position and Locality—Not uncommon in the fine olive slates of Llampeter, Pembrokeshire. 
2nd Ord. Dorstprancniata (Cuv.) 
This group is almost equivalent to the Annelida Errantia of authors; it is composed of centipede- 
like worms, having the body entirely naked, and possessed of great powers of locomotion; the head is 
distinct and provided with eyes, jaws, antennee, &e. The group is characterized by the almost uniform 
distribution of the gills in tufts over the remainder of the segments, which are very numerous and equal 
among themselves; one pair of gills on the outer edge of most of the rings; at the base of each branchial 
tuft is a retractile bunch of stiff bristles for locomotion, enclosed at the base in a thick, fleshy, lobe-like 
foot, and often a short, slender, fleshy, articulated process above and another below, called “ cirri.” 
The two Cambrian genera, Nercites and Myrianites, apparently the first created beings of their class, 
belong to this, the most highly organized, group of all the Annulata. 
Genus. NEREITES (Mac Leay). 
Gen. Char.—Body very long, of nearly equal width throughout, of upwards of one hundred nearly 
equal and similar segments; each segment with a thick, short, ovate, lobe-like foot, surmounted by a 
slender cirrus exceeding the foot and the segment in length (or the distance between one cirrus and 
the next). 
