2. EXTERNAL. MORPHOLOGY AND THE GENERAL BODY PLAN 



13 



The tentacles, anatomically part of the protosoma, are always remarkable 

 for their considerable length (Figs. 2, 7, 16). In number they vary from one to 

 200 or more. In view of the complexity of their structure and of their great 

 physiological importance they will receive separate treatment (pp. 25 to 79). 

 It should be noted that the Spirobrachiidae are distinguished from all 

 other Pogonophora by the development of a lophophore — a peculiar un- 

 paired outgrowth of the protosoma bearing the major part of the tentacles 

 (p. 27). 



The mesosoma, or second segment of the body, is cylindrical or slightly 

 flattened dorso-ventrally. Not infrequently its dorsal and ventral surfaces are 

 marked by median furrows. A most characteristic and invariable feature of 

 the mesosoma is the bridle or frenulum, represented by a pair of brown or 

 black (rarely paler) cuticular keels or crests developed in the fore-part of the 

 segment (Figs. 2-8, 15, 16). Beginning in the mid-ventral line the keels run 

 obliquely forward on to the lateral surfaces of the body, then on to the dorsal 

 side, gradually tapering as they do so. Usually they lie on more or less 



elevated epidermal ridges. In certain species 

 the thickened ventral ends of the keels of the 

 bridle do not come into contact, though they 

 lie close together (Siboglinum plumosum, S. 

 robustum, S. taeniaphormn, Polybrachia bar- 

 bata, Lamellisabella zachsi, etc), in others 

 they touch (Polybrachia annulata, Lamellisa- 

 bella johanssoni etc), and finally, in a third 

 group, they fuse together (Oligobrachia and 

 many species of Siboglinum). The dorsal ends 

 of the keels of the bridle are usually separated 

 from one another, but in Heptabrachia subtilis, 

 Spirobrachia grandis, Siboglinum frenigerum, 

 S. vinculatum and S. buccelliferum they 

 coalesce. 



It is difficult to determine the functional 

 significance of the bridle. I doubt whether it 

 serves to support the fore-part of the body 

 against the edge of the tube when the animal 

 is extended. The filmy, soft and very delicate 

 walls of the fore-part of the tube in the ma- 

 jority of Pogonophora (p. 120) render such 

 an interpretation doubtful. It is more likely 



Fig. 6. Front end of the body of 

 Siboglinum fedotovi from the 

 ventral side. 



c- cephalic lobe; «'-glandular 

 girdle; /-bridle; g/ - globular 

 gland seen by transparency; 

 ms - mesosoma ; ps - protosoma ; 

 s 1 - groove between the proto- 

 soma and the mesosoma ; s 3 - 

 pretentacular groove separating 

 the cephalic lobe from the pro- 

 tosoma ; / - tentacle. 



