200 UNSEGMENTED “WORMS.” 
two threads attached to its tip, the threads being retractor muscles. 
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 stinging cells or adhesive papille. There is a hint of 
a similar structure in some Turbellarians, and the organ may be inter- 
preted as one which was originally tactile, but which has become 
secondarily aggressive. It is protruded by the muscular contraction of 
the walls of the proboscis sheath, which forms a closed cavity surround- 
ing 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 heemo- 
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 ampullee 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 7etrastemma are protandrous. The 
organs consist of siniple 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 Zetrastemma, a species of Linews) 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. 
