SIPUNCULIDA 273 



trochophore stage indicates a close connection with the immediate ancestors 

 of that group. 



The Sipunculoidea are more or less elongated worms, the largest of 

 which are 20 cm. or more in length, which live in the sand or mud, either 

 free or in tubes or snail shells. The body is cylindrical and very con- 

 tractile, unsegmented and not ciliated, and without metameric ap- 

 pendages, spines, or bristles; it is made up of two divisions, a usually 

 slender anterior portion called the introvert, which can be invaginated, 

 and the thicker hinder portion. The subphylum contains two classes. 



Key to the classes of Sipunculoidea: 



Cj Body elongate ; anus at base of introvert ; tentacles usually present. 



1. SIPUNCULIDA 

 Oy Body robust ; anus at hinder end ; no tentacles 2. PBIAPULIDA 



CLASS 1. SIPUNCULIDA.* 



Elongated worms with short, hollow tentacles at the forward end 

 which are either distinct or more or less united and usually surround the 

 mouth, and with the anus in the dorsal surface near the base of the intro- 

 vert. The body wall consists of a cuticula, a single-layered hypodermis, 

 and the muscles. The hypodermis gives rise to large glands which lie 

 beneath it, in certain species enveloped in a gelatinous connective tissue 

 called the cutis, and open through the cuticula to the owter surface. The 

 muscles consist of an outer circular and an inner longitudinal layer and 

 between them delicate oblique muscles, which, however, are not present in 

 the introvert. The longitudinal muscles in certain species (Sipunculus) 

 are split up into regularly recurring bundles, which produce a lattice-like 

 effect (Fig. 446). The body cavity is voluminous and is bounded by a 

 ciliated peritoneal lining; it contains a corpusculated blood fluid. Two or 

 four retractor muscles extend back from the front end of the base of the 

 introvert, by means of which invagination is effected. The digestive system 

 consists of a narrow tube of about the same diameter throughout, which 

 extends from the mouth at the front end to near the hinder end of the 

 body, then turns on itself and extends forward to the anus. The two 

 limbs of the intestine are usually twisted spirally together, in certain 

 species around a single muscle strand called the spindle muscle. A blood 

 vascular system is present in form of an oesophageal ring canal, tentacular 

 canals which extend from the ring canal into the tentacles, and one or two 

 contractile ca3ca (hearts) which extend from the ring canal a short dis- 

 tance along the oesophagus. These organs contain a fluid which serves to 

 extend the tentacles, which are probably respiratory as well as sensory in 



* See "Die Sipunculiden," by E. Selenka, in Reisen im Arch. d. Philipp, von C. 

 Semper, 1883. "The Sipunculids of the Eastern Coast of North America," by J. H, 

 Qerould, Prqc, TJ, S. Nat. Mus., Vol.. 44, p. 373, 1913. 



