ON POLYPS. 



Fig. 10. 



communicating with that portion of the body which is lodged within 

 the tube, and resembling a second mouth. A remarkable action has 

 been observed to take place in these parts of the polyp, producing a 

 continual variation in their form ;* a fluid appears at intervals to be 

 forced from the lower compartment into the space intervening be- 

 tween the two rows of tentacula, which becomes gradually dilated 

 into a globular form (Jig. 10 ; 2 and 3.) This distension continues 

 for about a minute, when the upper part, contracting in turn, 

 squeezes back the fluid which fills it into the lower compartment 

 through the opening a, which then closes preparatory to a repeti- 

 tion of the operation. The intervals between these actions were, 

 in the specimen observed by Mr. Lister, very evenly eighty 

 seconds. In Tubularia indivisa the sheath or cell, &, which en- 

 closes the polyp, is perfectly diaphanous, allowing its contents to 

 be readily investigated under 

 the microscope. When thus 

 examined, a continual circu- 

 lation of particles was visi- 

 ble, moving in even, steady 

 currents in the direction of 

 the arrows (fig. 10; 1) along 

 slightly spiral lines represent- 

 ed in the drawing. The par- 

 ticles are of various sizes, 

 some very minute, others 

 apparently aggregations of 

 smaller ones ; some were 

 globular, but they had gene- 

 rally no regular form. In 

 fig. 3, d, a series of longitu- 

 dinal lines are perceptible, 

 which most probably are ovi- 

 gerous filaments, resembling 

 those of Tubipora musica. 



Actiniadce. The next family of polyps, from the fibrous 

 character which the substance of their bodies assumes, have been 

 named by zoologists " Fleshy Polyps.' 1 '' They differ indeed re- 

 markably from the soft gelatiniform structures which have hitherto 

 come under our notice, exhibiting traces of muscular fibre which 

 are not to be mistaken. 



* Lister, on the structure and functions of Tubular and Cellular Polypi. Philoso- 

 phical Transactions, 1834. 



