CILIA. 



611 



In the second form (Jig. 294) the stem and 

 Fig. 294. 



Campanularia. 



branches are formed externally of a tough 

 (generally horny) substance, and within this 

 of a transparent soft tissue, which is tu- 

 bular and contains a granular matter. The 

 polypi resemble hydra ; each is lodged in 

 a horny cell (a, a), from which it partially 

 protrudes itself; one orifice surrounded with 

 tentacula serves both for receiving aliment 

 and discharging faeces ; this leads to a stomach 

 (6), which communicates through an opening 

 (c) at the bottom of the cell with the interior 

 of the tubular stem and branches, the attached 

 part or base of the polype being continuous 

 with the soft internal tube, of which the po- 

 lypes might be regarded as a prolongation. 

 In this form of polype, which exists in most 

 true species of Sertularia, Campanularia, and 

 Plumularia, and in allied genera, the tentacula 

 or arms are destitute of cilia and incapable 

 of giving an impulsion to the water. But a 

 very remarkable motion has been observed by 

 Cavolini* and Mr. Listerf in the granular 

 matter contained in the stem and branches. 

 Although this motion has not been traced to 

 the agency of cilia, yet as it is connected with 

 our subject, I shall briefly notice it here. 

 When the stem and branches of the above- 

 named zoophytes are examined with a high 

 magnifying power, a current of granular par- 

 ticles is seen running along the axis of the 

 tube. The current, which is compared to the 

 running of sand in a sand-glass, after con- 

 tinuing" one or two minutes in the same direc- 

 tion, changes and sets in the opposite one, 

 in which it continues about as long, and again 

 resumes the first, thus alternately flowing 

 along the stem to the extremities of the 

 branches, and back again. The change of 

 direction is sometimes immediate, but at other 

 times the particles are quiet for a while, or exhibit 

 a confused whirling motion for a few seconds 

 before the change takes place. Mr. Lister has 

 discovered that the currents extend into the 

 stomachs of the polypi, in which and in the 



Fig. 295. 



13 



* Memorie per servire alia storia de' Polypi 

 Marini, p. 121 and 197 ; p. 56 and 91 of the Ger- 

 man Translation. 



t Phil, trans. 1834, p. 369. 



mouth a remarkable agitation of particles is 

 perceptible. When these particles are allowed 

 to escape from a cut branch, they exhibit, 

 according to Mr. Lister, something very like 

 spontaneous motion. The immediate cause of 

 these currents is not apparent ; it seems not to 

 be muscular contraction of the tube ; perhaps, 

 like the agitation within the stomach, they 

 may be owing to internal cilia. As to their 

 use Mr. Lister supposes the circulating matter 

 " to be a great agent in absorption, and to 

 perform a prominent part in the obscure pro- 

 cesses of growth ; and its flow into the stomach 

 of the polypi seems to indicate that in this 

 very simple family (the Sertulariffi) it acts also 

 as a solvent of the food." Page 77. Perhaps 

 the polypi of the Pennatula and Virgularia 

 should be referred to this head. In these 

 Dr Grant* discovered a constant vibratory 

 motion within the mouth, apparently pro- 

 duced by cilia placed round the entrance ot 

 that passage, and he saw minute particles oc- 

 casionally propelled from the mouth. Their 

 tentacula, as in the zoophytes last referred to, 

 did not excite currents. 



The third form of polype is found in 

 Tubularia. Fig. 295 represents a magnified 

 view of a common species, 

 the Tubularia indivisa. There 

 is a transparent horny tube 

 (a, a), containing a soft mat- 

 ter, which at the extremity of 

 the tube is continuous with 

 the stomach (6) and the mouth 

 (c). There are two rows of 

 tentacula or arms, one (d) 

 immediately surrounding the 

 orifice of the mouth, the 

 other (e) further back, be- 

 tween the mouth and stomach. 

 The arms are destitute of 

 cilia and excite no movement 

 in the water; but Mr. Listerf 

 has discovered a remarkable 

 motion of particles within the 

 tube, which has some re- 

 semblance to the circulation 

 of globules observed in plants 

 of the genus C/iaru. These 

 particles moved in a current 

 within the tube, the general 

 course of the stream being 

 parallel to the slightly spiral 

 lines of spots on the tube, 

 and in the directions marked 

 by the arrows. On the greater 

 part of the side first viewed 

 (the one represented) it set as from the poly- 

 pus ; but on the other side the flow was to- 

 wards the polypus, each current thus occupy- 

 ing half the circumference. The tube had a 

 granulated appearance between the lines of 

 spots, and beneath this the particles ran. Their 

 course was even and uniform without any 

 starting or dancing motion, such as is observed 

 in the Sertularia?. At the nodous parts of the 



Tubularia 

 indiviia. 



* Edin. Plrl. Journ. 



t Phil. Trans. 1834, p. 366. 



