xv KEY TO SIPUNCULOIDEA 425 



caecum. Hooks usually present. Four retractors (in Ph. Rupellii 



only two?). Heart almost always without caeca. Eye-spots 



always present Eggs oval, flat, reddish. Almost entirely small 



tropical species . . . .1. PHYMOSOMA 



B. Body devoid of papillae. Tentacular membrane surrounds the 



mouth in a circlet. Rectum with one or more caeca (except S. 



edulisl). Hooks absent except in S. australis. Eggs spherical. 



The tentacular membrane contains a vascular network. A ventral 



contractile vessel usually present in addition to the heart. Mostly 



large forms. Found in all seas . . 2. SiPOXcuLus 



II. The longitudinal muscles in the body-wall form a continuous sheath, 



and are not split up into bundles. 



A. Two brown tubes. Numerous tentacles form a wreath round 



the mouth. Alimentary canal forms a complete spiral, free 

 behind except in Ph. Hanseni. Spindle-muscle usually present. 

 One or more ligaments present, but only on the anterior convolu- 

 tions of the intestine. Adhesive papillae always absent Hooks 

 very frequently absent. Eggs spherical. Found in all seas. 



3. PHASCOLOSOMA 



B. Two free brown tubes. Only four or six plumed tentacles. 



A complete intestinal spiral, not attached behind. Spindle- 

 muscle always present. One or more ligaments present, but only 

 on the anterior convolutions of the intestine. Hooks are present, 

 but sometimes fall off early in life. Heart usually bears caeca. 

 Found only in the tropics. . . .4. DENDROSTOMA 



C. Only one brown tube, that of the right side, present ; it is 



attached to the body-wall throughout its entire length. Numer- 

 ous tentacles form a circle round the mouth. The alimentary 

 canal forms no spiral, or an incomplete one. No spindle-muscle, 

 but the intestine is attached to the body-wall throughout its 

 length by numerous ligaments. Adhesive papillae often present. 

 Not more than two retractors. Spherical eggs. Inhabits Mollusc 

 shells or tubes. Found in all seas . . 5. PHASCOLION 



III. At both ends of the trunk a distinct horny shield, or tube-like cornifica- 

 tion, or a calcareous ring at the anterior end of the trunk. Hooks 

 sometimes present. Longitudinal muscles continuous or split up 

 into bundles. 



A. A shield at both ends of the trunk. Introvert excentric, arising' 



from the ventral side of the anterior shield. Tentacles small 

 and few in number, arranged in a horse-shoe above the mouth. 

 A spindle- muscle, which arises from the posterior end of the 

 body, traverses the intestinal coil. Two retractors only, these 

 are the ventral ; they are frequently fused together from their 

 point of origin. ... 6. ASPIDOSIPHON 



B. A calcareous ring surrounds the anterior end of the trunk, from 



the middle of which the introvert is extruded. Longitudinal 



muscles continuous. Hooks bifid. Tropical. 7. CLOEOSIPHON 



0. A corneous ring, from which the introvert issues, surrounds the 



anterior end of the trunk, and the posterii r end of the trunk is 



