21G ROBERT PAYNE BIGELOW ON 



Development of the Tentacles. — During the third day the peristome appears as a minute 

 ridge surrounding the posterior end of the hirv^a, a short distance from the mouth. The 

 first series of tentacles arises during the following twentj'-four hours as four perradial 

 angles in the margin of the peristome (Fig. 8) . Four interradial tentacles appear almost 

 simultaneously with these, or sometimes considerably later (Figs. 9, 10, 11, and 12). 

 The elongation of the tentacles takes place rapidly, so that at about the end of the sixth 

 day the peristome is surmounted by a crown of eight tentacles, whicli equal the jiroboscis 

 in length. 



With the broadening of the peristome the differentiation of the body of the scyphis- 

 toma into stem and calyx becomes apparent externally (Figs. 1.3 and 14). When the 

 eight perradial and interradial tentacles have become long enough to reach some distance 

 beyond the mouth, eight adradial tentacles appear in the angles between them. In 

 Fig. 14 the adradial tentacles are distinctly developed, and two of the tentacles of the first 

 series are bifurcated near the base. Figs. 15 and 16 represent the tyjjical scyphistoma in 

 the sixteen-tentacle stage. The tentacles are now long and graceful, and thickly dotted 

 with batteries of nettle cells. 



When fully developed, the scyphistoma is about one millimeter and a half in diameter ; 

 and it is provided typically with thirty-two tentacles.^ But there is as much variation in 

 the number of tentacles in the scyphistoma as there is in the number of sense organs and 

 parameres in the adult. The number of tentacles is seldom less than thirty-two, often 

 greater. The way in which this variation takes place is indicated in Figs. 21 and 14. 

 All degrees of anomaly maybe observed, from a bifurcated tentacle shown at x in Fig. 21, 

 through the condition represented in Fig. 14, to two completely separated tentacles occu- 

 pying the position of one typical one. 



The tentacles of a well-developed scyphistoma, when fully expanded, exceed the 

 length of the body several times. Accoi-ding to the position in which they are held, the 

 tentacles may be divided into two series. Those of one series are held nearly erect, while 

 those of the other series, consisting of the alternate tentacles, are bent backward until 

 their tips nearly touch the ground vipon whicli the animal rests. The action of the ten- 

 tacles in capturing prey may be observed in a small aquarium, under the microscope. As 

 soon as a tentacle comes into contact with a small floating body, such as a copepod, it is 

 whipped quickly into the mouth, and at the same instant the side of the mouth toward 

 the tentacle is opened more widely. On one occasion I saw two tentacles make captures 

 at the same time, and the mouth expanded in both directions at once, showing a close 

 co-ordination between the movements of the tentacles and of the mouth. 



' Specimens have been foiind witli twenty-four tentacles, but it is uncertain wliether tliis is a regular stage in develop- 

 ment between the sixteen- and thirty-two-tentacle stage, or a duplication of tentacles of the eai-lier stage. 



