GENERAL ACCOUNT OF NEMERTEA. 173 



or spaces, a median and two laterals, which unite anteriorly 

 and posteriorly, and also communicate by numerous trans- 

 verse 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 haemo- 

 globin may be present. 



The excretory system is not fully known, but 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 ampullse 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 CEeca, 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 

 (Prosorhochjnus, a fresh-water Tetrastemma, a species of Linens) 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 in- 

 direct. 



( 1 ) In Cerebratidus ', 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. (2) In Linens 

 there is a sedentary larva, which has been interpreted as a reduced 

 Pilidium, and is known as the " larva of Desor." (3) In Hoplonemertea 

 the development is direct without metamorphosis. 



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 ccelom at least in 

 the larva. The recent discovery of flame cells in connection 

 with the excretory system confirms the idea of Platyhelminth 

 affinities ; but it is to be noticed that apart from the points 

 mentioned above, the reproductive system is strikingly 

 different. Professor Hubrecht has suggested that Nemer- 

 tines exhibit affinities with Vertebrates, comparing proboscis 

 sheath with notochord, and so forth. 



