ON PHYMOSOMA. VAUIANS. 85 



right and left retractor muscles (figs. 2, 3, and 9). It usually 

 extends about 4 cm. behind the head, and it ends blindly behind. 

 Anteriorly it opens in the middle ventral line into the smaller 

 or tentacular horseshoe, and at the point of junction is a large 

 sinus which surrounds about three quarters of the brain — in 

 fact, all those parts which are not in contact with the epidermis 

 (figs. 2, 4, and 8). The nervous matter is thus in close contact 

 ■with the blood, being separated only by a thin layer of con- 

 nective tissue, and the endothelium of the blood-space (fig. 27). 

 The walls of this third part or dorsal vessel are muscular, 

 and in some specimens are much contracted and crumpled. 

 This vessel appears to serve as a reservoir for the corpusculated 

 fluid, and when it contracts and the fluid is forced forward, it 

 would serve to evert the lip and extend the tentacles. The 

 whole of this space is lined by flat epithelium. I ha'^e never 

 seen cilia on the walls, and it is entirely closed. 



The Nephridia. 



The nephridia or the renal organs are in the form of a 

 single pair of " brown tubes," as in other Sipunculidge. They 

 lie on either side of the middle ventral line at some little dis- 

 tance from the nerve-cord. Their anterior extremities, near 

 which are the external openings, being a little anterior to the 

 level of the anus (fig. 3). 



Each nephridium is about 1 cm. long, the length in preserved 

 .specimens varying according to the space of contraction of its 

 muscular coat ; by means of this muscular layer the whole 

 organ has the power of shortening and dilating, and also of 

 throwing itself into a number of curious curves. 



At the anterior extremity is a dilated bladder, the diameter 

 of which is from four to five times that of the posterior cellular 

 portion of the organ. The internal opening is situated at the 

 anterior extremity of the bladder and is provided dorsally with 

 a prominent ciliated lip' (fig. 18). The external orifice is just 



1 The existence of this opening is doubted by Selenka, 'Die Sipunculiden,' 

 but it is sufficiently obvious in all the specimens. It was demonstrated iu 

 another species of Phyniosoma by Dr Spengel. 



