274 SIPUNCULIDA 



function. A pair of nephridia, called the brown tubes, opens to the out- 

 side near the anus. The nervous system consists of a cerebral ganglion 

 on the dorsal side of the oesophagus, which is connected by a pair of 

 commissures with an unsegmented ventral nen'e; this nerve passes to the 

 hinder end of the body, sending off numerous nerves which are not regu- 

 larly paired. A pair of simple eyes, pigmented or not, lies within the 

 brain. A ciliated canal extending from the surface to the brain and called 

 the cerebral tube, the walls of which are pigmented in certain species, 

 is present just back of the tentacles : tactile organs are also often present. 

 The sexes are separate, but alike in appearance. A pair of gonads de- 

 velops in the peritoneum upon the base of the retractor muscles which 

 discharge their products into the body cavity, whence they make their way 

 to the outside through the nephridia. The young animal passes through 

 an active free-swimming stage. The metamorphosis is not accompanied 

 by any well-marked evidences of metamerism, and the adult worm is still 

 trochophore-like, indicating that the animals are very primitive forms 

 near the base of the annelid stem. The animals live in the sand and mud, 

 which they swallow in large quantities. They are found in almost all 

 parts of the world, mostly in shallow water. The class contains about 11 

 genera and over 100 species, 16 of which have been found on the east and 

 4 on the west coast of this country. 



Key to the genera Sipunculida here described : 



ai Longitudinal muscles divided into bundles (except Phascolosoma gouldi). 

 &i Tentacular fold instead of tentacles; no papillae on trunk.. 1. Sipunculus 

 62 Isolated tentacles present. 



Ci Tentacles encircle the mouth 2. Siphonosoma 



c'2 Tentacles in a crescent dorsal to mouth 3. Physcosoma 



Oa Longitudinal muscles not split into bundles (with Phascolosoma gouldi). 



61 Worms free-living with numerous tentacles 4. Phascolosoma 



62 Worms inhabit tubes or shells 5. Phascolion 



1. Sipunculus L. Mouth surrounded by a fluted tentacular fold, 

 without isolated tentacles, behind which is the cerebral tube; no hooks on 

 the introvert; longitudinal muscles in 17 to 41 distinct bundles, giving a 

 lattice-like effect; 4 retractor muscles; rectum with 1 or more caeca; 2 

 contractile hearts : 16 species, mostly of large size, in most seas. 



S. nudus* L. (Fig. 445). Body up to 21 cm. long; the anterior 

 sixth covered with papillae; 13 longitudinal muscles: Beaufort, North 

 Carolina; Key "West; Europe. 



2. Siphonosoma Spengel. Similar to Sipunculus but with integu- 

 mental blind sacs and a statocyst near the tentacles; cerebral tube a shallow 

 pit: several species. 



* See "Ou Some Points on the Anatomy and Histology of Sipunculus nudus L.," 

 by H. B. Ward, Bull. Mus. Comp. Zool., Vol. 21, p. 143, 1S91. 



