242 THE RELATIONS OF PROTOZOA TO HYDROZOA. 



Fig. 140rr. 



gitudinal tubes seem to empty. Such is one of the curious fal- 

 lacies into which naturalists have fallen in their desire to homol- 

 ogize in a special manner the organization of the Scyphostoma 

 with that of the full-grown Aurelia. 



Now when we trace the development of the tentacles, we find 

 that they arise at points which have definite relations with the 

 position of the four equidistant pilasters (a). In the first stage 

 of growth, after the egg-phase, the embryo has a cylindrical 

 form, like this, (fig. 140a,) and at one end a simple 

 aperture (m) for a mouth, which leads to an elongate 

 cavity (d) within. After swimming about for a while 

 by means of its vibratile cilia, it settles down on the 

 end opposite to the mouth, and becomes fastened to 

 whatever it rests upon. Soon after this, the four pi- 

 lasters, which I just spoke of, make their appearance 

 within the elongate cavity of the cylinder, and about the same 

 time, the tentacles begin to develop. At first, either two, as in 

 this figure, (fig. 1406, t,) or at most four tentacles, 

 spring up at as many alternate points with the 

 pilasters ; then four others, either consentaneously 

 or alternately, originate opposite the pilasters ; 

 and the succeeding ones, in successively overlap- 

 ping circles, and alternating with those which 

 have preceded them, develop in the same way, 

 and eventually are merged into an apparently single circle, as I 

 have described in Tubularia. 



There are, however, certain Hydroids, whose tentacles are un- 

 questionably arranged in a spiral ; for instance, those of Rhizo- 

 geton (p. 73, fig. 38) and Coryne (p. 78, fig. 42). But the spiral 

 in this case has a different relation to the organization from 



Fig. 140a. The planula stage of Aurelia flavidula. Per and Les. 100 diara. 

 m, the mouth ; d, the cavity of the body. The body is covered by vibratile 

 cilia. Original. 



Fig. 140&. The primary, or bitentaculate stage of the Scyphostoma of Aure- 

 lia flavidula. Per and Les. 100 diain. s, the stem; m, the mouth; d, the di- 

 gestive cavity ; <, the tentacles. Original. 



Fie. 1406. 



