CHAPTER XII. 



SIPUNCULOIDEA* (GEPHYREA ACHAETA). 



Unsegmented vermiform animals with a spacious coelom, antero- 

 dorsal anus, and one pair of nephridia. The anterior part of the 

 body is invagindble. 



As stated before on p. 451, we have thought it necessary, in the 

 present state of our knowledge, to break up the old group Gephyrea 

 into (1) the Echiuroidea (Gephyrea armata) which remains with 

 the Annelida, (2) the Sipunculoidea (Gephyrea Achaeta), (3), the 

 Priapuloidea, and (4) Phoronidea (Gephyrea tubicola}. The three 

 last of these we advance provisionally to phyletic rank, not being 

 able to subordinate them to any other group. The Sipunculoidea 

 and Phoronidea are undoubtedly Coelomata; but of the Priapuloidea 

 we cannot say this, and the fact that we deal with them in the 

 part of the work devoted to coelomate animals must be regarded 

 as a concession to the older view of their relationship to the 

 Sipunculoidea, rather than as an expression of opinion on our part 

 that they have a coelom. 



The Sipunculoidea are elongated, vermiform animals and live in 

 sand and ooze in the sea; they also bore in coral rock. The 

 anterior part of the body is of a different appearance to the 

 posterior or main portion; it is called the introvert, because it 

 can be invaginated into the larger posterior portion by means 

 of special retractor muscles. The mouth is placed at the anterior 

 end of the introvert and is in relation with a row of ciliated, 

 hollow tentacles, which may be arranged in a circle or in the 

 form of a double horseshoe, the concavity of which is dorsal. 



* Keferstein, "Beitragez. Anat. u. syst. Kenntnissd. Sipunculiden,"^./. w. Z., 

 xv., 1865. E. Seleuka, "Die Sipunculiden," Semper 's Reisen, 1883. D. C. 

 Danielssen and J. Koren, " Gephyrea," The Norwegian N. Atlantic Expedition, 

 1876-78, 1881. B. Hatschek, "Ub. Entwick. v. Sipunculus midus," Arb. Zool. 

 List. Wien, 5, 1884. A. E. Shipley, " On Phymosoma varians," Q. J. M. S., 31. 



