10 SIDERASTREA RADIANS. 



in most members of the inner cycles becomes bifurcated a little above midway, 

 each half terminating in a light-colored knob which represents a battery of 

 nematocysts ; the outermost members, however, are simple, like those of the 

 majority of coral polyps.* 



In studying the internal anatomy of the polyp, it is found that the bifur- 

 cated members communicate with the entocoelic chambers, and the simple 

 forms with the exoccelic chambers. Development shows that the double 

 tentacles are at first simple, and such may be the condition of some of the 

 outer entocoelic in nearly mature polyps. The number of tentacles always 

 corresponds with the number of septa, the bilobed tentacles being situated 

 over the entosepta, and the simple tentacles over the exosepta. Their order 

 of appearance is described later in connection with the development of the 

 larval polyps. 



The tentacular walls are smooth, that is, devoid of the nematoblast 

 tubercles which so frequently characterize the tentacles of coral polyps. 

 The aggregation of large nematoblasts is restricted to the apical knobs. 

 The stems are hollow and brown in color, the latter character due to the 

 presence of Zooxanthellae in the endodermal tissues. The knobs, on the 

 other hand, are solid and colorless. The organs are adhesive and able to 

 take particles of food from the tips of forceps. There is little difference in 

 size among the tentacles of the various cycles, but the bifurcated members 

 are larger than the simple. 



During retraction, and even sometimes on full expansion of the polyp, 

 the tentacles appear as unstalked, spheroidal tubercles, sessile on the disc. 

 In the case of the bilobed tentacles, a knob is situated on each side of the 

 septum with which the tentacle corresponds, while in the simple tentacles the 

 knob is directly over the septum (plate 6, fig. 32). No stem can be recog- 

 nized under such circumstances, and sections show that it is not invaginated, 

 but has become part of the discal wall (plate 7, fig. 40). Such a condition 

 of the tentacular stems is often found in corals, but rarely among the larger 

 tentacles of actinians. When the tentacles are fully expanded they stand 

 obliquely to the surface of the disc, and are bent outwardly. The organs 

 have never been found invaginated, a phenomenon which frequently occurs 

 in coral polyps and alcyonarians. 



* Verrill (1901, p. 154) makes the following remarks with regard to the tentacles of S. radians : " the 

 tentacles are small, short, cylindrical, orclavate; they form several circles, and appear somewhat scattered 

 those of successive cycles being in different circles and decreasing in size. But t/iey are not bilobed, nor 

 trilobed, as Agassiz and Pourtales supposed. This appearance is due to a smaller one standing on one 

 or both sides of a larger one, and close to it." From the part italicized it would appear that the tentacles 

 in Verrill's Bermudan specimens must have been altogether different from those in Jamaica polyps, 

 which, moreover, agree with those observed by Agassiz and Pourtales. 



