THE GEPHYKEA OF JAPAN. 77 



Key to the Genera and Species of Gephyrea treated 

 of in this Paper. 



SipuncuJoUJea, 



I. The longitudinal muscle of the body-wall is divided into J.)-ol 

 .se|)arate bundles. Two or four retractor muscles present. 



A. — Body covered with well develi:)[)ed papilla?. Hooks usually 

 present on the introvert. Filamentous tentacles arranged in 

 a single semicircle above the mouth. Retractor muscles com- 

 monly in two })airs (except in two s|)ecies, PJuiniosonn' oiio- 

 mlchianum and Fh;/inostuua iKiI/aciisr, both which are cliarac- 

 terised by having only two muscles). No rectal gland, no 



ventral ves'-el. Two distinct eye-spots on tlie ganglion 



Geiiits Phymosoma. R 78. 



B. — Body devoid of papill;o. Tentacles tilamentous or flap-like, 

 in the latter case they spring from the tentacular membrane. 

 Hooks usually absent. Rectum with one or more cteca. 

 Usually a ventral vessel in addition to the dorsal vessel. 

 No eye-spot Genus Sipunculus. R 79. 



II. The longitudinal muscle of the body-wall is perfectly continuous. 



A. — Two segmental organs and a si)indle muscle present. 



a. — Numerous tentacles. »Spindle muscle is sometimes 

 fixed posteriorly to the body-wall. T^sually two 



pairs of retractor muscles 



Genus Phascoi.osoma. R. ~i\). 



b. — Tentacles s[)ring from foar main stems. Hooks, if 

 present, large, but few and scattered. Spindle 

 muscle posteriorly free from the body-wall. Onl}^ 



one ]»air (ventral) of retractor ujuscles 



(reiius Denduostoma. I'. 8U. 



B, — Only one segmental organ ; spindle muscle wanting. 



c. — Numerous tentacles arranged in a circle round the 

 mouth. Two retractor muscles. The worms be- 

 longing to this genus inhabit Molluscan shells 



Genus 1'hascoijox. R. 80. 



