1 88 UNSEGMENTED "WORMS" 



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 hemo- 

 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 Tetrastemuia 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 excretion. 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 Platyhelminth 

 affinities ; but it is to be noticed that the reproductive 

 system is strikingly different. Professor Hubrecht has 

 suggested that Nemertines exhibit affinities with Verte- 

 brates, comparing proboscis sheath with notochord, and 

 so forth. This seems far-fetched. 



Classification. 



Order Palceonemertea. Mouth behind brain ; no stilets ; outer 

 circular muscles, inner longitudinal muscles, and sometimes 

 a third inner circular layer ; lateral nerves either outside 

 muscular layers or embedded in longitudinal muscles. 



e.g. Carinella, Carinoma, Cephalothrix. 



Order Heteronemertea. Mouth behind brain ; no stilets ; three 

 layers of muscle, the innermost longitudinal ; lateral nerves 

 outside circular muscular layer. 

 e.g. Linens, Cerebratnhis. 



