ANATOMY OF SIPUNCULUS 



413 



one another like tiles on a roof. In some other genera, as 

 Phymosoma, the introvert bears rows of horny hooks, which are 

 apt to fall off as the animal grows old. 



The trunk has from thirty to thirty-two longitudinal furrows, 

 the elevations between which correspond with a similar number 

 of muscles lying in 

 the skin. This longi- 

 tudinal marking is 

 crossed at right angles 

 by a circular marking 

 of similar origin, the 

 elevations of which cor- 

 respond with the circular 

 muscles in the skin. 

 These two sets of mark- 

 ings thus divide the skin 

 of the trunk into a 

 number of small square 

 areas, very regularly 

 arranged (Fig. 212). 



The outline of the 

 trunk is more or less 

 uniform, but it is cap- 

 able of considerable 

 change according to the 

 state of contraction of 

 its muscles. The cir- 

 cular muscles, for in- 

 stance, may be contracted 

 at one level, thus caus- 

 ing a constriction at this 

 spot. The colour of S. 

 nudus is a somewhat 

 glistening greyish-white. 



The anterior end of the fully-expanded Sipunculus may be 

 termed the head ; here the skin is produced into a frayed fringe 

 which stands up in the shape of a funnel round the mouth. 

 This fringe is grooved on its internal surface with numerous little 

 gutters, all of them lined with cilia, which by their constant 

 motion keep up a current which sweeps food into the mouth. 



Fig. 211. Right half of the anterior end of Sipun- 

 culus nudus L., seen from the inner side and 

 magnified, a, Funnel-shaped grooved tentacular 

 crown leading to the mouth ; b, oesophagus ; 

 c, strands breaking up the cavity of the tenta- 

 cular crown into vascular spaces ; c', heart ; d, 

 brain ; e, ventral, and e', dorsal retractor muscles ; 

 /, ventral nerve-cord ; G, vascular spaces in ten- 

 tacular crown. 



