The name Archi-Annelida was employed originally by Hatschek as an ordinal name for the Polygordiidae, a family with a single genus, Polygordius, to which he added subsequently the genus Protodrilus. Foettinger has more recently still added a second family, Histriodrilidae, represented by the parasitic Histriodrilus Benedeni (= Histriobdella Homari). All the Archi-annelids are marine; all have a small pro- and a large peri-stomium; the somites of the body are similar to one another, especially in the Polygordiidae, and are but feebly marked externally. There are no setae, no parapodia, cirri or branchiae, but the head carries two or more tentacles. The muscles form a single layer of longitudinal fibres disposed in four bands, two dorso-lateral, two ventro-lateral, the latter widely separate. Polygordius Villoti is stated by Perrier to have circularly disposed fibres. The nervous system is retained in the hypo-dermis. A pair of ciliated grooves is present on the head. The oesophagus is restricted to the peristomial somite, and there is a simple intestine. A muscular sub-oesophageal bulb or proboscis lies beneath the oesophagus and at the posterior margin of the mouth in Histriodrilus and Protodrilus. Simple nephridia are present.

Histriodrilus and Polygordius are of separate sexes; Protodrilus is hermaphrodite.

The Histriodrilidae are the higher group, inasmuch as there are individual nerve ganglia and special (?) genital ducts. The Polygordiidae have an intra-muscular nerve-plexus connected to the cells of the hypo-dermis, and oblique dorso-ventral muscles are present in each somite, as in many Polychaeta. Polygordius, like some Nemertea, is apt to break into fragments when irritated or injured.

Histriodrilus Benedeni ( - Histriobdella Homari) is parasitic upon the Lobster, the eggs of which it devours. It is a minute worm 14 mm. long. There is a distinct head, a body composed of three narrow somites, a sexual enlargement which appears to include two (? 3) somites followed by three short somites. The head carries a median and two laterally placed pairs of sensory tubercles, and a pair of short limbs. The last somite carries a larger pair of posterior limbs. The hypodermis is composed of nucleated protoplasm. The nervous system consists of cerebral ganglia, oesophageal commissures, and a pair of ventral cords which fuse in each somite to form a ganglion. Organs of special sense are the sensory setae of the cephalic tubercles, and of a similar tubercle attached to each posterior limb. The digestive tract is ciliated and consists of an oesophagus, stomach-intestine and short rectum. The mouth is ventral, the anus terminal; both are surrounded by cilia. The oesophageal bulb is armed with three chitinoid jaws. The three first somites of the body possess each a pair of ciliated nephridial tubes. A fourth pair occurs close to the third in the female, but close to the ventral genital opening in the male, which has also a -fifth pair in the first of the three posterior somites.

The sexes are separate. Testes and ovaries are formed by the growth of splanchnopleural coelomic epithelium. The genital ducts in the female are a pair of wide ciliated tubes opening each into a dilatation in which sperm has been observed, and this in its turn into a second dilatation. The external apertures are lateral like the nephridial, and the hypodermic cells surrounding them enlarged. The latter probably secrete the egg-shell. The male appears to possess a pair of lateral evaginable penes. A median ventral genital opening leads into a canal which bifurcates, each of its branches ending in a vesicle. Sperm has been observed in these vesicles. Internal openings to the male ducts have not been observed. Vascular system absent (?).

Foettinger, Archives de Biol. v. 1884.

The family Polygordiidae includes the two genera Protodrilus and Polygordius. The species of the former genus are of minute size, provided with a longitudinal ventral and ciliated furrow, a muscular sub-oesophageal bulb, and are hermaphrodite. The best known is P. Leuc-karti from Messina, which is about 4 mm. long. The segmentation of the body is marked only by fine lines, by bands of cilia and internal septa; the last somite carries a pair of terminal glandular adhesive lobes. The head has a pair of long tentacles or palpi. Cilia are found on the tentacles, ventral aspect of the head, in the ventral groove, as a double prae-oral band, and five simple post-oral cephalic bands, as well as two bands, one at each end of every somite. The nervous system consists of a cephalic mass, two oesophageal commissures, two ventral cords connected from place to place by transverse fibrous commissures, but separated by the ventral groove. A gradual transition between ganglion cells and ordinary hypodermis cells is observable. The mouth is ventral, the anus terminal. The digestive tract is ciliated, and consists of oesophagus with sub-oesophageal bulb and intestine. There is a dorsal and ventral blood-vessel. A contractile bulb developed on the former drives the blood into the tentacles.

The afferent tentacular vessel is contractile, the efferent not so. The latter fuses with its fellow to form the ventral vessel. Lacunae in the walls of the intestine put the dorsal and ventral vessels in communication. The blood is feebly red when in mass. There is a pair of nephridia in each fully developed somite, which traverse the septa as in Oligochaeta and open laterally. The seven first somites contain paired ovaries, the next succeeding testes.

Two eye-specks and otocysts are stated to be present in P. (=- Poly-gordms)flavocapitatus, and P. Schneideri. The former is also said to have the coelome traversed by a network of connective tissue fibres, and a special oviducal aperture in the last somite.

The species of the genus Polygordius are larger than the species of Protodrilus. P. lacteus is 40 mm. long, P. Villoti 1 dm. The cephalic tentacles are short, and at the apex of the prostomium. The posterior somites are distinctly delimited. P. lacteus has the anus surrounded by eight tooth-like processes, and a short distance in front of it the body is begirt by a circle of twenty-four adhesive papillae. The cerebral ganglion in P. neapolitanus is divided into three lobes, anterior, middle and posterior. The oesophageal commissures originate from the middle lobe, and are united on the ventral aspect by a transverse commissure. From this point onwards there is a single median cord due to the fusion of the two larval cords. Remnants of the ciliated longitudinal furrow which divides the two cords of the larva are to be found within this cord as fine canals. The mouth is ventral; there is an oesophagus and ciliated intestine, the latter constricted by the transverse septa. The blood-vascular system consists of a dorsal vessel which gives origin in each somite to a pair of transverse vessels. The latter end caecally with the exception of the first pair which are united ventrally. The blood is red. There is a pair of nephridia in each somite.

The sexes are separate, and the genital products are developed in the posterior somites.

Perrier states that in P. Villoti the vicinity of the mouth, together with a small part of the last somite are ciliated, that a ventral vessel is probably present, and that the genital products escape by the nephridia.

Protodrilus appears to develope direct (Repiachoff). Polygordius, on the contrary, passes through a metamorphosis. The larva is a Trochosphere, and the body grows as does that of an ordinary Chaetopod. There is a pair of remarkable head-kidneys; each consists (Fraipont) of a horizontal ciliated intracellular tube terminating in a ciliated enlargement. A vertical branch soon appears similarly terminated. The enlargements multiply by division, and there are usually two to the vertical, three to the horizontal branch. Each of them carries several radiating caecal tubules representing aborted 'nephridial canals of the second order,' etc. (Fraipont). The tubules in P. neapolitanus are supported by a funnel-shaped membrane

According to Hatschek a longitudinal canal is formed in connection with each cephalic kidney. Segmental nephridial funnels develope in each somite in continuity with this canal, which then disappears, each nephridium opening externally by its own pore. The young Polygordius has a pair of eye-specks.

Species of Protodrilus and Polygordius, Fraipont, Archives de Biologie, v. 1884, note p. 250; the names as given by Fraipont have been followed here.

Protodrilus Leuckarti, Hatschek, Arb. Zool. Inst. Wien, iii. 1880. P. pur-pureus, Schneider, Archiv f. Anat. und Physiol. 1868, p. 56. P. flavocapitatus, Uljanin, apud Hoyer, Z. W. Z. xxviii. 1877, p. 389; development of, Repiachoff, Z. A. iv. 1881. P. Schneideri, Langerhans, Z. W. Z. xxxiv. 1880, p. 125.

Polygordius lacteus, Schneider, loc. cit. supra. P. Villoti, Perrier, C. R. 80, 1875. Development, Hatschek, op. cit. supra, i. 1878; of head, Id. op. cit. vi. (1), 1885; Fewkes, Bull. Mus. Harvard, xi. (No. 9, 1883), p. 195; head-kidney, Fraipont, op. cit. supra.

Nervous system of Archi-annelida, Fraipont, op. cit. supra. This author is stated to be preparing a monograph of the genus Polygordius for 'The Fauna and Flora of the Gulf of Naples.'