spermatozoa, when mature, present no peculiarities. As the ova are
in many species deposited in a gelatinous tube secreted by the body-walls,
in which they are arranged (three or more together) in flask-shaped
cavities, impregnation must probably take place either before
or at the very moment of their being deposited. The exact mode has
not yet been noticed.
Prosorhocmus claparedii is a viviparous form.
 |
|
Fig. 20.—Anterior end of a Carinella, partly diagrammatic. Magnified. (From Bürger.) 1, Opening of proboscis; 2, cephalic glands running to frontal organ; 3, dorsal commissure of brain; 4, cerebral organ; 5, upper dorsal nerve; 6, under dorsal nerve; 7, rhynchocoelic blood-vessel; 8, fore-gut; 9, rhynchocoel; 10, nerve to proboscis; 11, proboscis; 12, genital sac; 13, genital pore; 14, mid-gut; 15, circular nerves; 16, pore of excretory system; 17, lateral organ; 18, excretory canal; 19, lateral vessel; 20, lateral nerve; 21, oesophageal nerve; 22, mouth; 23, ventral ganglion of brain; 24, dorsal ganglion of brain; 25, rhynchodaeum. |
| |
| Fig. 21.—Pilidium-larva. | |
| B, | Bunch of cilia or flagellum.
|
| oe, | Oesophagus. |
| st, | Stomach. |
| cs, | Oesophageal outgrowth for lateral organ.
|
| am, | Amnion. |
| pr.d., Prostomial disk. | |
| po.d., Metastomial disk. | |
Development.—The embryology of the Nemertines offers some very remarkable peculiarities. Our knowledge of the development of the most primitive forms is scanty. Both Hetero- and Metanemertini have been more exhaustively studied than the other two groups, the first, as was noticed above, being characterized by peculiar larval forms, the second developing without metamorphosis.
The larva of Cerebratulus is called the pilidium. In exterior shape it resembles a helmet with spike and ear-lobes, the spike being a strong and long flagellum or a tuft of long cilia, the ear-lobes lateral ciliated appendages (fig. 21). It encloses the primitive alimentary tract. Two pairs of invaginations of the skin, which originally are called the prostomial and metastomial disks, grow round the intestine, finally fuse together, and form the skin and muscular body-wall of the future Nemertine, which afterwards becomes ciliated, frees itself from the pilidium investment and develops into the adult worm without further metamorphosis.
The eggs of these species are not enveloped by such massive gelatinous strings as are those of the genus Lineus. In the latter we find the young Nemertines crawling about after a period of from six to eight weeks, and probably feeding upon a portion of this gelatinous substance, which is found to diminish in bulk. In accordance with these more sedentary habits during the first phases of life, the characteristic pilidium larva, which is so eminently adapted for a pelagic existence, appear to have been reduced to a close-fitting exterior layer of cells, which is stripped off after the definite body-wall of the Nemertine has similarly originated out of four ingrowths from the primary epiblast. To this reduced and sedentary pilidium the name of “larva of Desor” has been given.
In the Metanemertini, as far as they have been investigated, a direct development without metamorphosis has been observed. It appears probable that this is only a further simplification of the more complicated metamorphosis described above.
As to the development of the different organs, there is still much that remains doubtful. The hypoblast in some forms originates by invagination, in others by delamination. The proboscis is an invagination from the epiblast; the proboscidian sheath appears in the mesoblast, but is perhaps originally derived from the hypoblast. The origin of the lateral organs has already been noticed; that of the nerve system is essentially epiblastic.
Affinities.—The position of the Nemertines in the animal kingdom is now looked upon as more isolated than was formerly thought, and recent writers have been inclined to treat them as a separate hylum. Whether this view be adopted or not, and whether the Turbellaria be regarded as nearly related or only remotely connected, there can be little doubt that the Nemertines resemble the Turbellaria more nearly than they do any other group of animals. Bürger even goes so far as to homologize the proboscis with the Turbellarian pharynx, and he sums up their relationship to the Annelids by the statement that to a certain extent the Nemertines represent Turbellaria which in the course of time have copied certain features of an Annelid character.
Literature.—J. Barrois, “Recherches sur l'embryologie des Némertes,” Annales des Sc. Naturelles, vi. (1877); O. Bütschli, “Einige Bemerkungen zur Metamorphose des Pilidium,” Archiv für Naturgeschichte (1873); L. von Graff, Monographie der Turbellarien (1882); A. A. W. Hubrecht, “Untersuchungen über Nemertinen a. d. Golf von Neapel,” Niederl. Archiv für Zoologie, ii.; Id., “The Genera of European Nemerteans critically revised,” Notes from the Leyden Museum (1879); Id., “Zur Anatomie u. Physiologie d. Nervensystems d. Nemertinen,” Verh. kon. Akad. v. Wetensch. (Amsterdam, 1880), vol. xx.; Id., “The Peripheral Nervous System of the Palaeo- and Schizonemertini, one of the layers of the Body-wall,” Quart. Journal of Micr. Science, vol. xx.; Id., “On the Ancestral Forms of the Chordata,” Ib. (July 1883); W. Keferstein, “Untersuchungen über niedere Seethiere,” Zeitschr. f. wissensch. Zool. vol. xii. (1863); J. von Kennel, “Beiträge zur Kenntniss der Nemertinen,” Arbeiten a. d. zool.-zoot. Instit. ii. (Würzburg, 1878); W. C. MacIntosh, A Monograph of British Annelida: I. Nemerteans (Ray Society, 1873-1874); A. F. Marion, “Recherches sur les animaux inférieurs du Golfe de Marseille,” Ann. des Sc. Nat. (1873); E. Metschnikoff, “Studien über die Entwickelung der Echinodermen und Nemertinen,” Mém. de l'Acad. Imp. de St Petersb. xiv. (1869); Max Schultze, Beiträge zur Naturgeschichte der Turbellarien (Greifswald, 1851) and Zeitschr. für wissensch. Zool. iv. (1852), p. 178; W. B. Benham, Quart. Journ. Micr. Sci. xxxix. (1896), p. 19; A. Brown, Proc. Roy. Soc. lxi. (1897), p. 28; O. Bürger, Zeit. f. wiss. Zool. l. (1890), p. 1; Id., Mitt. Zool. St Neapel, x. (1891), p. 206; Id., Zeit. f. wiss. Zool. liii. (1892), p. 322; Id., Verh. Deutsch. zool. Gesellsch. (1893); Id., Fauna u. Flora d. Golfe d. Neapel, Monograph 22 (1895); A. Dendy, Proc. Roy. Soc. Victoria (n.s.), iv. (1892), p. 85, v. p. 127 (1891-1892); B. Haller, Arb. Zool. Inst. Wien, viii. (1889), p. 276; A. A. W. Hubrecht, “Challenger” Reports, xix. (1887); L. Joubin, Arch. Zool. Exper. (2), viii. (1890), p. 461; Id., “Nemertines,” in Blanchard’s Traité de zoologie (1894); J. N. Lebedinsky, Arch. Mikr. Anat. xlix. (1897), p. 503: T. H. Montgomery, Zool. Anzeig. xvii. (1894), pp. 298, 301; Id., Zeit. f. wiss. Zool. lix. (1895), p. 83; Id., Zool. Jahrb. (Anat.) x. (1897), p. 1; A. E. Verrill, Trans. Connecticut Acad. New Haven, viii. (1892), p. 382; D. Bergendal, Zool. Anzeiger, xxiii. (1900), p. 313; W. R. Coe, Zool. Jahrb. (Anat.) xii. (1899), p. 425; Id., Trans. Connect. Acad. ix. (1895), p. 479; Id., Proc. Wash. Acad. iii. (1901), p. 1; T. H. Montgomery, Journ. Morph., xiii. (1897) p. 381; Id., Zool. Jahrb. (Anat.) x. (1897), p. 265; R. C. Punnett, Quart. Journ. Mic. Sc. xliv. (1900), p. 111; Id., Willey’s Zool. Results, pt. v. (1900), p. 569; Id., Quart. Journ. Mic. Sc. xliv. (1901), p. 547; Staub, Semon’s Forschungsreisen (5 Bd., 1900); C. B. Thompson, Zool. Anzeiger, xxiii. (1900), pp. 151, 627; C. B. Wilson, Quart. Journ. Mic. Sc. xliii. (1900), p. 97. (A. A. W. H.; A. E. S.)
NEMESIANUS, MARCUS AURELIUS OLYMPIUS, Roman
poet, a native of Carthage, flourished about A.D. 283. He was
a popular poet at the court of the Roman emperor Carus
(Vopiscus, Carus, 11). He wrote poems on the arts of fishing
(Halieutica), aquatics (Nautica) and hunting (Cynegetica), but
only a fragment of the last, 325 hexameter lines, has been
preserved. It is neatly expressed in good Latin, and was used
as a school text-book in the 9th century. Four eclogues, formerly
attributed to Titus Calpurnius (q.v.) Siculus, are now generally
considered to be by Nemesianus, and the Praise of Hercules,
generally printed in Claudian’s works, may be by him.
Complete edition of the works attributed to him in E. Bährens,
