88 



NEMERTINES 



being characterized by peculiar larval forms, the second 

 developing without metamorphosis. 



The larva of Cerebratulus is called the pilidiuin. 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. 19). It encloses the primitive alimentary tract. 

 Two pairs of invaginations of the skin, which originally 



FIG. 19. Pllidium larva. B, bunch of cilia or flagellum; , oesophagus ; st, 

 stomach; cs, cesophageal outgrowth for lateral organ; am, amnion; pr.d., 

 prostomial disk; po.d., metastomial disk. 



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 developes into the adult worm 

 without further metamorphosis (2, 13). 



The eggs of these species are not enveloped by such 

 massive gelatinous strings as are those of the genus Linens. 

 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, appears to have been reduced to a 

 close-fitting exterior layer of cells, which is striped off 

 after the definite body-wall of the Nemertiue 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 (1). 



In the Hoplonemeriea, as far as they have been investi- 

 gated, a direct development without metamorphosis has 

 been observed. It appears probable that this is only a 

 further simplification of the more complicated metamor- 

 phosis 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 delamina- 

 tion. 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. 



Literature. 



(1) J. Barrois, " Reeherches sur 1'embryologie des Nemertes," 

 Annales cles Sc. Naturelles, vi., 1877. 



(2) O. Biitschli, " Einige Bsmerkungen zur Metamorphose des 

 Pilidium," Archivfiir Naturgeschichlc, 1873. 



(3) L. von Graff, Monographic der Turbellarien, 1882. 



(4) A. A. W. Hubreeht, " Untersuchungen iiber Nemertinen 

 a. d. Golf von Neapel," Niederl. Archivfur Zoologie, ii. 



(5) Id. , " The Genera of European Nemerteans critically revised," 

 Notes from the Leyden Museum, 1879. 



(6) Id., "Zur Anatomic u. Physiologic d. Nervensystems d. 

 Nemertinen," Verh. Kon. Akad. v. Wetensch., Amsterdam, 1880, 

 vol. xx. 



(7) Id., "Tlie Peripheral Nervous System of the Palseo- and 

 Sehizonemertini, one of the layers of the body-wall," Quart. 

 Journal of Micr. Science, vol. xx. 



(8) Id., "On the Ancestral Forms of the Chordata," Ib., July 

 1883. 



(9) \V. Kefersteiu, " Untersuchungen iiber niedere Seethiere," 

 Zeitschr. f. wissensch. Zool.,^ vol. xii., 1863. 



(10) J. von Kennel, " Beitritge zur Kenntniss der Nemertinen," 

 ArbeUen a. d. zool.-zoot. Instit., ii., Wiirzburg, 1878. 



(11) W. C. Macintosh, A Monograph of British Annellida : I. 

 Nemcrteaus, Ray Society, 1873-74. 



(12) A. F. Marion, " Reeherches sur les animaux inferieurs du 

 Golfe de Marseille," Ann. des Sc. Nat., 1873. 



(13) E. Metschnikoff, ' ' Studien iiber die Entwiekclung der 

 Eehinodermen und Nemertinen," Mem. de VAcad. Imp. de St. 

 Petcrsb., xiv., 1869. 



(14) Max Sclmltze, Seitriigc zur Naturgeschichte der Turbellarien, 

 Greifswald, 1851, and Zeitschr. fur wissensch. Zoo!., iv., 1852. 



(A. A. W. H.) 



