ANNULOSA : NEMERTIDA. 



241 



are mostly in separate individuals, and the generative organs 

 have the form of sacs placed between the muscular walls of 

 the body and the digestive canal, and discharging their con- 

 tents by lateral pores. 



Reproduction takes place by the formation of true ova, by internal gem- 

 mation, or by transverse fission. In Nemertes, however, the egg gives rise 

 to a larva, from which the adult is developed in a 

 manner closely analogous to that described as char- 



acteristic of the Echinodermata. The larval form 

 of Nemertes was described by Johannes Miiller, 

 under the name of Pilidium (fig. 117.4). It is "a 

 small helmet-shaped larva, with a long flagellum 

 attached like a plume to the summit of the helmet, 

 the edges and side-lobes of which are richly cili- 

 ated. A simple alimentary canal opens upon the 

 under surface of the body between the lobes. In 

 this condition the larva swims about freely ; but, 

 after a while, a mass of formative matter appears 

 on one side of the alimentary canal, and elongat- 

 ing gradually, takes on a worm-like figure. Event- 

 ually it grows round the alimentary canal, and 

 appropriating it, detaches itself from the Pilidium 

 as a Nemertid provided with the characteristic 

 proboscis, and the other organs of that group of 

 Tnrbellaria " (Huxley). Whilst some Nemertids 

 are thus developed by internal budding from a 

 ciliated provisional larva or "nurse," others exhibit 

 no such phenomena, but are directly developed into 

 the adult form, without undergoing any striking 

 metamorphosis. 



Whilst the Nemerteans undoubtedly show very 

 close affinities to the Planarians, they are also 

 nearly related to the Annelida proper, both as 

 regards their general form and their internal struc- 

 ture. One of the most remarkable of the links 

 between these two groups is to be found in Balano- 

 glossus, the systematic position of which is still 

 uncertain. This extraordinary worm is found 

 burrowing in the sand in the Mediterranean and 

 along the eastern coasts of North America, rang- 

 ing to depths of 2500 fathoms, and possesses a 

 flattened worm-like body, terminating in front in 

 a protrusible hollow proboscis. The external in- 

 tegument is ciliated, as in the Nemerteans, and 



there are no setiform organs of locomotion, such as are found in so many 

 Annelides. A water- vascular system is present ; there is a well-developed 

 alimentary canal ; and the process of respiration is carried on by means 

 of a singular branchial sac formed by a chitinous framework with ciliated 

 apertures, somewhat similar to the "branchial sac" of an Ascidian ; whilst 

 the sexes are in different individuals. The most remarkable point, how- 

 ever, about Balanoglossus is its mode of development. The larva is a 

 free-swimming, barrel-shaped, ciliated body, which was originally discovered 

 by Miiller, and described by him under the name of Tornaria, as the 

 young of some Star-fish. In spite of its close resemblance to the larva; of 



Q 



Fig. 1 1 8. Morphology of 

 Nemertida. Prorhyn- 

 chus Jluviatilis ' o 

 Mouth ; c Ciliated 

 grooves (sense-organs ?); 

 s Spine, attached behind 

 to (/O the sac of the pro- 

 boscis.; ce Gullet ; g Gas- 

 tric glands ; z" Intestine ; 

 ov Ovary. The proboscis 

 in this form is very small. 

 (After Gegenbaur.) 



