200 UNSEGMENTED " WORMS." 



drawn in by two threads attached to its tip, the threads being re- 

 tractor muscles which arc fastened posteriorly to the wall of the pro- 

 boscis sheath. But in front of the attachment of the retractor muscles 

 there is a non-eversible glandular region which secretes an irritant fluid. 

 In many cases there are stilets at the tip of the eversible portion, and if 

 these be absent, there are adhesive papillae. There is a hint of a 

 similar structure in some Rhabdocoel Turbellarians, and the organ may 

 be interpreted as originally tactile, secondarily aggressive. It is pro- 

 truded by the muscular contraction of the walls of the proboscis 

 sheath, which forms a closed cavity surrounding the proboscis, and 

 containing a fluid with corpuscles (Fig. 103). 



In the majority there are three longitudinal blood vessels or spaces, 

 a median and two laterals, which unite anteriorly and posteriorly, and 

 also communicate by numerous transverse branches. The vessels or 

 spaces are remnants of a ccelom. The blood is a colourless fluid, 

 sometimes at least with nucleated elliptical corpuscles in which hoemo- 

 globin may be present. 



The excretory system usually consists of two coiled ciliated canals 

 opening in the anterior region by a varying number of ducts. They are 

 said to divide up internally into numerous fine branches ending in flame- 

 cells, or in blind ampullae embedded in the walls of the blood vessels. 



The sexes are usually separate, and the reproductive organs are 

 always simple. A few species (of Geonemertes and Prosadenophorus] are 

 hermaphrodite, and some species of Tetrastemma are protandrous. The 

 organs consist of simple sacs, arranged in a series on each side between 

 the intestinal caeca, and communicating with the exterior by fine pores. 

 The ova are often laid in gelatinous tubes, and are probably fertilised 

 shortly before or at the time of expulsion. In three or four forms 

 (Prosorhochmus > a fresh- water Tetrastemma^ a species of Lineus] known 

 to be viviparous, the fertilisation must, of course, be internal. 



Segmentation is total and almost always equal ; a complete or partial 

 gastrula is formed, and development may be direct or indirect. 



In Cerebratulus, etc., the larva is adapted for pelagic life, and is 

 known as the Pilidium. " In external 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" (Hubrecht). 

 Out of this, somewhat abruptly, the adult form arises. 



Relationships. The Nemertines are probably nearly related to 

 Turbellaria, but show some very distinct marks of advance. Of these, 

 the most noticeable are the presence of an anus, of a closed vascular 

 system, of a coelom at least in the larva. The presence of flame-cells 

 in connection with the excretory system confirms the idea of Platy- 

 helminth affinities ; but it is to be noticed that the reproductive 

 system is strikingly different. Professor Hubrecht has suggested that 

 Nemertines exhibit affinities with Vertebrates, comparing proboscis 

 sheath with notochord, and so forth. 



Classification. 



Order Protonemertini. Brain and lateral nerves outside the 

 muscular layers ; mouth behind brain ; no stilets. 

 Carinella, Hubrechtia. 



