424 GEPHYREA 



tube are, as a rule, inhabitants of tubes or shells, and do not 

 move actively about in the sand. 



The eggs of all members of the family, with the exception of 

 the genus Phymosoma, are spherical, but those of the last-named 

 genus are elliptical. They are always surrounded by a thick 

 membrane, the " zona radiata," pierced by numerous pores. 



Aspidosi})hon (Fig. 215) is easily recognised by the presence 

 of two symmetrically-arranged cuticular shields, one at each end 

 of the trunk. These are formed by the fusion of minute cuti- 

 cular plates, such as exist in the skin of most Sipunculids. The 

 posterior shield is radially symmetrical, but the 

 anterior is somewhat like the shell of a Pecten, 

 and symmetrical only about one plane. The 

 introvert is protruded from the acute angle 

 of the anterior shield, and when extended lies 

 almost at right angles to the trunk, instead of 

 being, as is usually the case, in the same 

 straight line with it. In many specimens, and 

 these seem as a rule to be the older ones, a 

 deposit of calcium carbonate takes place over 

 these shields, covering over and concealing their 

 external markings. 



Cloeosiplwn {Ecliinosiplion^ has a calcareous 

 ring, consisting of four or five rows of lozenge- 

 ".;/>/! n',~ tfunmtti's shaped calcareous bodies forming a close mosaic, 

 Kef. x 2. , in- arranged round the base of the introvert, which 



trovert partially , -i i j i i j. i 



extended, but not ^heii extended is in the same straight line as 



sufficiently to show the trunk. Each piece bears a brown spot, 



which is said to be the pore of a gland (Fig. 



217). Golfiny-ia Lankester, has a cylindrical horny thickening at 



the anterior end of the trunk and another at the posterior. 



Key to the Genera of Sipunculoidea. 1 



I. The longitudinal muscles in the body-wall divided into 17-41 distinct 



bundles. Four retractor muscles. 



A. Body covered with papillae. Numerous filiform tentacles which 

 seldom (or never?) surround the mouth, but stand above and 

 dorsal to it in a horse-shoe, with the opening dorsal. No rectal 



1 Sclenka, iJie HijmnculidcH. Semper' s Jleiscn, i>/i Arcliipcl d. I'fiillqnnen, vol. 

 iv. 1883. 



