536 SIPUNCULOIDEA. 



the terminal portion (rectum) short and straight, leading to the anus. 

 The intestine passes back to the hind end of the body and then turns 

 forwards, the two limbs being coiled spirally round one another. 

 Delicate muscular bands pass from the body-wall to the intestine, 

 and there is often a muscle the spindle muscle which extends 

 along the axis of the intestinal spiral from the hind end of the 

 body to the rectum. There are no special glands, but a caecal 

 diverticulum, which varies much in size in different individuals 

 and opens into the rectum, is often present. Two tufts of tissue 

 are also attached to the rectum ; they were formerly supposed to 

 be vestiges of the anal vesicles of the Ecliiuroidea. The alimentary 

 canal particularly the intestinal portion is generally full of sand, 

 excepting along a ciliated groove, which extends along its whole 

 length. 



The retractor muscles are usually four in. number ; they are 

 inserted into the alimentary canal just behind the mouth, and arise 

 from the body-wall a little behind the anus (Fig. 433). 



The nervous system consists of a single ventral cord without 

 ganglionic swellings, and showing no sign of being formed of two 

 halves ; it divides below the oesophagus into two cords which embrace 

 the oesophagus and unite in a single supra-oesophageal ganglion on 

 the dorsal side just behind the attachment of the retractor muscles. 

 The cord and ganglion give off numerous nerves to adjacent parts. 

 In the dorsal middle line, outside the tentacular circlet, there is 

 an ectoclermal pit which reaches as far as the supra-oesophageal 

 ganglion ; it is called the cerebral organ, and is lined by ciliated cells. 

 In Physcosoma (Phymosoma) the inner end of this pit is bilobed and 

 lined with cells containing a black pigment ; it is embedded in the 

 substance of the supra-oesophageal ganglion. 



The body-cavity is entirely coelomic and very spacious ; it contains 

 a richly corpusculated fluid, which often has a pink colour. The cor- 

 puscles are of two kinds biconcave discs, which contain the red 

 pigment called haemerythrin, and amoeboid cells. In addition there 

 are found floating in the coelomic fluid the reproductive cells, and in 

 Sipunculus and Phascolosoma some peculiar ciliated structures called 

 "urns" ; these are apparently budded off from the coelomic epithelium 

 overlying the dorsal blood-vessel. 



The vascular system is closed, and contains a corpusculated fluid ; 

 it consists of a contractile vessel lying on the dorsal side of the 

 oesophagus, and of an annular vessel surrounding the mouth. The 

 dorsal vessel ends blindly behind, and in front opens into the circum- 



