POLYDORA CILIATA. 201 



in the group, and in most preparations each forms a convex anterior margin on each side 

 of the prostomium, whilst in a dorso- ventral view a constriction posteriorly indicates the 

 limit of the segment. The mouth opens on the ventral surface immediately behind the 

 notched rostrum. 



The movements of the tentacles (Plate XCIX, figs. 1 and 1 a) of this species are 

 most interesting. Thus, in an example, the tabes of which projected from a sandstone 

 covered with Lepraliae, Spirorbis, and small zoophytes— especially Olava multicornis — the 

 lashing hither and thither of these delicate filaments was in contrast with the quiet 

 exposure of the plumes of Spirorbis, or the rapid fan-like action of the minute Balani. 

 Not only are these organs thrown whip-like in various directions, but each undergoes a 

 series of vermiform movements in addition, and as no part of the body or head of the 

 annelid is visible, they resemble a struggling Ascaris held by the middle and endeavouring 

 to escape. After lashing the water one or both may bend toward the rock-surface, pro- 

 ceeding rapidly with a serpentine movement, searching in a curious manner any irregu- 

 larity of the surface. The object of this is apparent when the tentacle meets with a 

 loose particle of sand or mud. The tentacle does not contract, but with a vermiform 

 motion it remains more or less extended, whilst the particle passes along the surface of 

 the organ to the mouth of the tube. If suitable it enters the mouth of the tube, and 

 by-and-by is pushed out along with the head of the annelid to be arranged in the circum- 

 ference of the tube. The proximity of other tubes causes the tentacles of neighbouring 

 Polydorae frequently to come into collision, especially when each seizes the same particle 

 of sand or mud, and the stronger bears off the particle. These tentacles (that is their 

 papillae) thus exercise a sucker-action as well as being endowed with cilia, since they 

 adhere to the plumes of Spirorbis and retain a sand-particle in struggling with rival 

 tentacles. Their restless movements, indeed, cause trouble to neighbouring forms such 

 as Spirorbis, Balani, and the zoophytes. Polydora seldom protrudes its head, and on the 

 slightest and most ordinary occurrence, such as a tentacle coming in contact with a branchia 

 of Spirorbis, it is rapidly withdrawn. These external tubes appear to be useful in 

 enabling the annelid to take more advantage of the surrounding water whilst its delicate 

 body is protected. 



When the fresh tentacle is examined, as, for instance, by seizing it in full extension 

 at the tube (Plate XCIX, fig. 1 a), the contracted organ presents a large brownish-green 

 blood-vessel in its centre, and a ciliated groove on its inner surface from base to apex. 

 The tip is provided with distinct papillae, and these organs also occur throughout the 

 entire length of the groove. Such a structure explains the seizure and retention of a 

 particle as well as its transit from the tip to the base, and, further, leaves the tentacle at 

 liberty to proceed with its captures without having to bend toward the mouth. The 

 muscular power of the tentacles is great, without reference to their almost ceaseless 

 waving. Occasionally a small mass of mud and sand travels outward along the tentacle, 

 and is dropped from its extremity. When removed from its tube the tentacles are much 

 contracted and often coiled like rams' horns, the whole organ being transversely frilled 

 and wrinkled, but the inner groove remains distinct, and for fully three-fourths of its 

 length it is richly ciliated. 



The body is largest in the anterior third, flattened dorsally and rounded ventrally, 



149 



