ANNULOIDA : SCOLECIDA. 1 67 



the so-called " prae-oral region," or " prostomium ; " and this is 

 often modified into a singular protrusible and retractile organ, 

 called the " proboscis," the exact use of which is not known. 

 The mouth opens into a muscular pharynx, which is often evert- 

 ible ; and the intestine may be either straight or branched, 

 but always terminates caecally behind, and is never provided 

 with an anal aperture. The " water-vascular system " commu- 

 nicates with the exterior by two or more contractile apertures. 

 The nervous system consists of two ganglia, situated in front 

 of the mouth, united by a commissure, and giving off filaments 

 in various directions. Pigment-spots, or rudimentary eyes, 

 from two to sixteen in number, are often present, and are al- 

 ways placed in the prae-oral region of the body. The male and 

 female organs are united in the same individual, and the pro- 

 cess of reproduction may be either sexual, by means of true 

 ova, or non-sexual, by internal gemmation or transverse fission. 



The Planarians have been divided into two sections, as 

 follows : 



Section A. RHABDOCCELA. Intestine straight, not branched. 

 Body elongated, rounded, or oval. 



Section B. DENDROCCELA. Intestine branched or arborescent 

 Body flat and broad. 



SUB-ORDER II. NEMERTIDA. The Nemertida, or " Ribbon- 

 worms," agree in most essential respects with the Planarida. 

 They are distinguished, however, by their elongated, vermi- 

 form shape, by the presence of a distinct anus, by the posses- 

 sion of a distinct perivisceral cavity, by the absence of an ex- 

 ternal aperture to the water-vascular system of the adult, and 

 by the fact that the sexes are distinct. The Nemertida further 

 differ from the other Platyelmia in possessing a pseudohaemal 

 system in addition to, and distinct from, the water-vascular 

 system. 



Reproduction takes place by the formation of true ova, by 

 internal gemmation, 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 

 characteristic of the Echinodermata. The larval form of Ne- 

 mertes was described by Johannes Miiller, under the name of 

 Pilidium (fig. 48, 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 ciliated. 

 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, elon- 



