NEMERTEA. 179 



mentary system. The proboscis is a hollow muscular 

 structure, very richly innervated, and is sometimes protruded 

 with such force that it separates from the body, and then 

 " often retains its vitality for a long time, apparently crawl- 

 ing about as if it were itself a worm " (Hubrecht). It has 

 been compared in its retracted state to a glove finger drawn 

 in by a thread attached to its tip, the thread being the 

 retractor muscle. But in front of the attachment of the 

 retractile muscle there is a non-eversible glandular region 

 which secretes an irritant fluid. In many cases there is a 

 stylet at the tip of the eversible portion, and if this be absent, 

 there are stinging cells or adhesive papillae. There is a hint 

 of a similar structure in some Turbellarians, and the organ 

 is usually interpreted as one which w r as 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 containing fluid and 

 surrounding the proboscis. (Fig. 57, P.s.) 



Vascular System. 



In the majority there are three longitudinal blood vessels 

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

 anteriorly and posteriorly, and also communicate by numerous 

 transverse vessels. The vessels or spaces are remnants of a 

 coelome. The blood is a colourless fluid, sometimes at 

 least with corpuscles in which haemoglobin may be present. 



Excretory System. 



In most, if not all, there are two coiled nephridia, 

 one on each side of the oesophagus opening anteriorly. 

 (Fig. 57, E.p.) 



Reproductive System. 



The sexes are usually separate, and the reproductive 

 organs are always simple. They consist of simple sacs, 

 arranged in a series on each side between the intestinal 

 coeca, 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 extrusion. In three 

 or four forms known to be viviparous the fertilisation must, 

 of course, be internal. 



