496 ZOOLOGY sect. 



are separate, the gonads simple, and the nephridia act as 

 gonoducts. 



The larva is in most cases a trochophore, and may develop a 

 metameric segmentation which disappears as development pro- 

 ceeds. 



Order 1. — Inermia (Sipunculoidea). 



Gephyrea with an introvert and usually tentacles or a tentaclar 

 fold. The anus is dorsal. Seta? are absent. Nephridia a single 

 pair, or absent altogether. 



Order 2. — Armata (Echiuroidea). . 



Gephyrea with an elongated prostomial proboscis. The anus 

 is posterior. Two or more seta?. A single nephridium, or two 

 or three pairs of nephridia. 



Systematic Position of the Example. 



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

 Sipunculus differs from other genera of the family Sipunculidcc 

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

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

 Sipunculidw is one of two families of the order Inermia ; and differs 

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

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

 appendages 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 Chajtopoda. 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, seta: are 

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

 in parapodia ; in Bonellia there is only a single pair, situated about 

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

 (Fig. 391), 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, 



