There are two principal longitudinal and lateral spaces in Carinella, Cephalothrix, and Carinoma; there is added to these in Polia, Valeneinia, and Schizonemertea, a median vessel placed above the digestive tract arising from an anterior lacunar connection between the two lateral spaces beneath the sheath of the proboscis, and partly imbedded in the lower wall of that sheath. The Hoplonemertea possess a looped vessel in the head, which passes anteriorly above the sheath of the proboscis, posteriorly below it, and at this point is connected with three longitudinal vessels, a median, dorsal, and two lateral. Transverse loops connect the three vessels in the region of each dorso-ventral set of muscles in Polia, Valeneinia, all Schizonemertea, and most Hoplonemertea. The lateral vessels in the body lie below the level of the lateral nerves as a rule. In Malacobdella the vessels give off branches, especially in the head and posteriorly into the sucker. The blood spaces are filled with a liquid containing, in some cases at least, corpuscles, which are oval and red with haemoglobin in Drepanophorus. The existence of nephridia has not been demonstrated in Cephalothrix and some other Nemerteans. They are present, however, in many, probably in all.
They are two in number, one on each side, placed anteriorly. Each organ consists in Carinella of a tube lying close to the blood-vessel, into which it opens in front and behind, and of a spongy (?) gland lying in the wall of the blood-vessel and connected from place to place with the tube. The nephridial tube of Carinoma has three connections to the blood-vessel. It is itself lined by glandular cells. In other Nemerteans it is wholly independent of the blood-spaces, though it may lie in the lacuna at the side of the oesophagus in Schizonemertea. It is branched to a greater or less degree in the Hoplonemertea. The nephridial cells are ciliated in Carinoma, Valencinia, and possibly in others. There is a single duct to each organ in Carinella, Carinoma, Cerebratulus, Langia, most Hoplonemertea; a large number in Polia, Valencinia,- Lineus, Amphiporus lactifloreus. The duct, whether single or multiple, opens above the level of the lateral nerve.
1Salensky regards it from its mode of development as the homologue of the proboscis of some Turbellaria, which is really the invaginated apex of the body. See papers on Pilidium and Monopora, cited p. 641.
The sexes are separate in nearly all instances. Geonemertes palaensis, Tetrastemma ( = Borlasia) hermaphroditica, T. (=B.)Kefersleinii are hermaphrodite. The sexual glands are placed in a series on each side of the intestine, one in each set of dorso-ventral muscles. They have at first no external opening, but during the evolution of the genital products approach the surface and finally open by a dorsally directed pore. Monopora vivipara, Prosorhochmns Claparedii, Tetrastemma obscurum, are viviparous forms. The ova are contained in capsules and laid in mucus in some Schizonemertea. Development is direct in Cephalothrix, viviparous forms, and Hoplonemertea; or in Schizonemertea accompanied by a metamorphosis. The genus Lineus has a creeping ciliated larva, the larva of Desor\ others have a ciliated pelagic helmet-shaped larva, the Pilidium. 1
The majority of Nemertea are marine. Many are capable of swimming. Pelagonemertes is pelagic. Two or three fresh-water forms are known, e.g. Tetrastemma aquarum dulcium from N. America; and four terrestrial, Tetrastemma agricola (Bermudas); T. Rodericanum (Rodriguez Is.); Geonemertes chalicophora (? Australia); G. palaensis, Pelew Is. Nemertes caret-nophila, and Cephalothrix Galatheae live on crabs, Malacobdella in the pallial cavity of various marine Lamellibranchiata; they are usually considered to be parasitic. The larger Nemerteans feed on fixed tubicolous Chaetopoda. All are probably carnivorous. Most members of the class have the power of breaking themselves into fragments if irritated. The Schizonemertea when thus self-mutilated or otherwise injured are able to reproduce the head, or to develope a head in connection with a fragment.
1 The larva of Desor is probably not so primitive a form as the Pilidium. The latter, according to Salensky, has an apical groove in which the ectoderm cells are thickened, probably representing the apical thickening of the Trochosphere. It has a provisional nervous system along the ciliated edges of the side lappets, besides various muscles. It has an oesophagus and stomach. In Desor's larva and the Pilidium alike, the larval epiblast is discarded after it has given origin to the ectoderm of the adult. As to the origin of the principal organs, the nervous is of ectodermic origin (Salensky), of mesodermic (Hubrecht). The ciliated cavities of the brain are derived from invaginations of the primary epiblast, which close and afterwards acquire an opening. The two oesophageal outgrowths which were supposed to give origin to the ciliated cavities, probably form the nephridia (Hubrecht). The proboscis is formed by an ingrowth at first solid (Monopora), or an invagination of the secondary (Salensky) or primary (Hubrecht) epiblast.
Its muscular walls and sheath are formed from the same mesoblastic rudiment; the proboscis cavity by a split in the rudiment (Salensky); or the cavity represents a part of the archicoele (note, p. 636), and the muscular walls and sheath have separate mesoblastic rudiments (Hubrecht). The oesophagus of the adult is the first part of the tract in the larva; the intestine as far as the anus its second part, the two being separate (?) parts of the archenteron in the larva of Desor. The generative organs are perhaps derived from the epiblast.
The Nemertea are classified by Hubrecht as follows
1.Palaeonemertea: no deep lateral fissure on the side of the head. No stylet in the proboscis. Mouth behind the ganglia. Cephalothrix, Carinella, Carinoma, Polia, Valencinia.
2. Schizonemertea: a deep longitudinal lateral fissure on each side of the head, from the bottom of which a ciliated duct leads into the posterior lobe of the ganglion. Lateral nerves between the longitudinal and inner circular muscular coat of the body-wall. Nervous tissue deeply tinged with haemoglobin. Mouth behind the ganglia. Lineus, Borlasia, Cerebratulus, Langia.
3. Hoplonemertea: one or more stylets in the proboscis. Mouth generally situated before the ganglia. Lateral nerves inside the muscular coats of the body-wall. No deep longitudinal fissures on each side of the head. Amphiporus, Akro-stomum, Drepanophorus, Tetrastemma, Oerstedia, Nemertes, Geonemertes, Proso-rhochmus, Malacobdella.
Nemertines, Hubrecht, Encyclopaedia Brit. (ed. ix.) xvii. Id. Revision of genera, Notes from Leyden Museum, i. 1879; ii. 1880. Monograph of British Annelids, pt. i., Nemerteans, Mcintosh (Ray Soc), 2 vols. 1873-74.
Carinoma, Oudemans, Q. J. M. xxv. 1885 (Suppl.), p. 7. Malacobdella, von Kennel, Arb. Zool. Zoot. Inst. Wurzburg, iv. 1877-78. Monopora (= Borlasia) vivipara, Salensky, Archives de Biol. v. 1884. Pelagonemertes, Moseley, A. N. H. (4), xv. and xvi. 1875.
Freshwater genera. Tetrastemma aquarum dulcium, etc, Silliman, Z. W. Z. xli.
1885, p. 70.
Terrestrial genera. Geonemertes chalicophora, Graff, M. J. v. 1879; G. pala-ensis, von Kennel, op. cit. supra. Tetrastemma Agricola, von Willemoes-Suhm, A. N. H. (4), xiii. 1874. T. rodericanum, Gulliver, Ph. Tr. 168, 1879.
Anatomy and physiology, Hubrecht, Niederland. Archiv f. Zool. ii. 1874; cf. Q. J. M. xv. 1875; Id. Z. A. ii. 1879; Dewoletzky, Z. A. iii. 1880. Nervous system, ciliated grooves and canals, Hubrecht, Q. J. M. xx. 1880. Circulatory and excretory systems, Oudemans, Q. J. M. xxv. 1885, Suppl. (with lit). Cf. Ancestral Form of Chordata, Hubrecht, Q. J. M. xxiii. 1883.
Development of Lineus obscurus (= Desor's larva), Hubrecht, Q. J. M. xxvi.
1886. Structure and metamorphosis of Pilidium, Salensky, Z. W. Z. xliii. 1886. Id. Monopora, supra.
Hubrecht is stated to be preparing a monograph of the Class in 'The Fauna and Flora of the Gulf of Naples.'