412 GEPHYREA 



interior ; or are provided with a long flexible but non-retractile 

 proboscis. The latter is easily cast off. They usually bear 

 spines or hooks of a hard chitinous character, secreted by the 

 epidermis or outermost layer of cells. The mouth is at the base 

 of the proboscis or at the end of the protractile part, the anus is 

 at the other end of the body or on the dorsal surface. The 

 nervous system consists of a ring round the mouth and of a 

 ventral nerve -cord. A vascular system is present as a rule. 

 Nephridia are found which act as excretory organs, and in most 

 cases also as ducts for the generative cells. The Gephyrea are 

 bisexual, and the male is sometimes degenerate. 



The group may be divided into four Orders : (i.) Sipuncu- 

 loidea ; (ii.) Priapuloidea ; (iii.) Echiuroidea ; (iv.) Epithetosomu- 

 toidea ; of these the first is by far the largest, both in number of 

 genera and of species. 



The Anatomy of Sipunculus nudus. 



External Characters. The body of S. nudus when fully 

 extended may attain a length of a foot, or even a little more ; in 

 this condition it is seen to consist of two portions, the anterior 

 of which is, however, retracted into the other when the animal is 

 disturbed. The retractile portion is sometimes termed the pro- 

 boscis, but as its nature is entirely different from that of the 

 proboscis of the Echiuroidea, it is better to refer to it as the 

 introvert. Special retractor muscles are attached on the one 

 hand to the body-wall about half-way down the body, and on 

 the other hand are fused into a muscular sheath which surrounds 

 the gullet, just behind the mouth. When these muscles con- 

 tract, they withdraw the introvert into the rest of the body or 

 trunk in much the same way as the finger of a glove may be 

 drawn into the hand, by a thread fastened to the inside of its 

 apex. The introvert is protruded by the contraction of the 

 circular muscles of the body-wall. These exert a pressure on the 

 fluid which fills the body-cavity, and by this means the sides of 

 the introvert are forced forward until finally the head is exposed. 



The introvert occupies about one -sixth or one -fifth of the 

 total body length. It is somewhat narrower than the trunk, 

 and is covered by a number of small flattened papillae, some of 

 which lie with their free ends directed backward, overlapping 



