INVERTEBRATES OF UNCERTAIN POSITION 



I8 7 



however, uncertain, and the affinities of the Gephyrea to one , 

 another are even doubtful. Consequently they have been 

 separated provisionally from the Annelida and divided into 

 three groups as follows: — 



(1) The Echiuroidea (Fig. 128) have traces of segmentation 

 in the adult, a proboscis (a), a pair of ventral hooked seta (b), 



Fig. 128. 



Fig. 130. 



Fig. i2g. 



Fig. 128. — Echiurus pallasii (of the group Gephyrea). a, mouth at the 

 end of the grooved proboscis; b, ventral hooks; c, anus. (From the Cam- 

 bridge Natural History.) 



Fig. 129. — Sipunculus nudus (of the group Gephyrea) laid open from the 

 side. A, anus; BD, brown tubes (nephridia); D, intestine; G, brain; Te, 

 tentacles; VG, ventral nerve-cord. (From Sedgwick, after Keferstein.) 



Fig. 130. — Priapulus caudatus (of the group Gephyrea). a, mouth 

 surrounded by spines. (From the Cambridge Natural History.) 



and a terminal anus (c). They usually live in crevices in rocks, 

 using their proboscis for locomotion, for capturing prey, and as 

 an organ of sense. There is a trochophore stage (p. 183) in 

 development. 



(2) The Sipunculoidea (Fig. 129) are unsegmented, with only 

 one pair of nephridia {BD), a large coelom, and an anus (A) on 



