PHYLUM ANNUL ATA 455 



The larva is in most cases a typical trochosphere, and may 

 develop a metameric segmentation which disappears as develop- 

 ment proceeds. 



ORDER 1. IKERMIA. 



Gephyrea with an introvert, and usually tentacles or a tentacular 

 fold. The anus is dorsal. Setae are absent. Nephridia a single 

 pair, or absent altogether. 



ORDER 2. ARMATA. 



Gephyrea with an elongated prostomial proboscis. The anus 

 is posterior. Two or more setse. A single nephridium, or two 

 or three pairs. 



Systematic Position of the Example. 



Sipunculus nudus is one of several species of the genus Sipunculus. 

 Sipunculus differs from other genera of the family Sipunculidce, 

 of which it is a member, mainly in having a tentacular fold around 

 the mouth, instead of a series of distinct tentacles. The family 

 Sipunculida: is one of two families of the order Inermia and differs 

 from the other, the Priapulidce, in the presence of either tentacles 

 or a tentacular fold at the oral, and the absence of filiform appen- 

 dages at the aboral end. 



3. GENERAL ORGANISATION. 



The Gephyrea are a class of worms whose position among the 

 Annulata is determined more from a consideration of their develop- 

 ment than of their structure in the adult condition, though the 

 latter suggests a tolerably close affinity with the ChsBtopoda. The 

 body of a Gephyrean is unsegmented, usually more or less com- 

 pletely cylindrical, broadest behind and narrowing towards the an- 

 terior end. The surface is covered with a chitinous cuticle developed 

 often into papilla?, or tubercles, or hooks. In the Armata, sctw are 

 present, but they are always very few in number and not implanted 

 in parapodia ; in Bone-Ilia there is only a single pair, situated about 

 the middle of the ventral surface ; in most species of Echiurus 

 (Fig. 357), in addition to this ventral pair, there are a number 

 arranged in one or two circlets around the posterior end. In the 

 Inermia the anterior part of the body is capable of being invagi- 

 nated within the part behind ; at the extreme anterior end of this 

 invaginable part or introvert, when it is evaginated, is the mouth 

 surrounded by a circlet of sometimes pinnate, sometimes simple, 

 tentacles, or by a lobed and plaited tentacular fold. The prostomium 

 is in such forms quite rudimentary- In the Armata there is no 

 introvert, but an elongated, highly contractile, simple or bifurcated 



