632 PROFESSOR E RAY LANKESTEU. 



The skeleton of each lophophoral arm consists of a portion 

 corresponding to the arm itself, which expands in the region 

 of the thorax into a thin plate, and attached to this — appa- 

 rently articulated to it — the skeletal axes of the two rows of 

 filaments. I was not able to detect any definite cell structure 

 in the skeletal tissue, but it has a refringency indicating a 

 certain density, and presents small twisted filaments and par- 

 ticles within its substance at intervals. It resists the action of 

 weak acids and alkalies. 



The existence of this skeleton is no doubt connected with 

 the very varied and free movement which each tentacle ex- 

 hibits, but the muscular cells related to it and to them cannot 

 be specially identified. In Professor McIntosh^s Cep halo- 

 discus dodecalophus (dredged by the "Challenger'^) I 

 have satisfied myself that a precisely similar skeleton exists. 

 The tentaculiferous arms of that form are extremely similar 

 in all respects to those of Rhabdopleura, excepting that they 

 are numerous instead of being limited to a single pair, and 

 have the free end terminating in a curious knob. 



The skeleton of the contractile polypide-stalk consists of an 

 axial cord (of the same tissue as that forming the lophophoral 

 skeleton), which extends from the sides of the abdomen, where 

 the stalk originates, to the termination of the soft portion of 

 the stalk, at the point where that organ is cuticularised, and 

 converted into Allman's blastophore. It is continued inde- 

 finitely along the cuticularised stalk in a modified condition, 

 being shrunken and histologically metamorphosed. In con- 

 nection with this skeletal cord it is easy to demonstrate the 

 presence of a muscular band consisting of closely-set fusiform 

 cells (PI. XL, fig. 5, c). This band lies on one side of the 

 skeletal cord, viz. on that furthest from the polypide's body. 

 Consequently, when it contracts it tends to throw the soft, 

 thin-walled stalk into coils. The extension of the polypide 

 from its tube necessitates a complete uncoiling and stretching 

 of the stalk, and is effected partly through the elasticity of the 

 skeletal cord straightening itself as the muscles relax, and 

 partly, as G. O. Sars has pointed out, by the active progressive 



