BROOKS AND RITTENHOUSE: ON TURRITOPSIS. 457 

 probably becomes fixed by a secretion extruded from the ectoderm 



cells along the whole length of its body. 



DEVELOPMENT OF THE HYDRANTH. 



Shortly after the larva becomes attached, a bud develops, usually 

 at about the center of the root, which is the beginning of the first 

 hydranth. Four small projections appear early around the distal 

 part of the bud; these will later form the first circle of tentacles. At 

 this time no mouth has yet developed. A young polyp in this stage 

 of development is shown in figure 51 of plate 35. The hydranth bud 

 continues to grow taller and after a few hours a second whorl of tenta- 

 cular buds is formed some distance below the first circle of tentacles. 

 When the polyp is from twenty to twenty-four hours old, or at about 

 seventy-two hours after the egg is laid, it is ready to develop the third 

 whorl of tentacles. Thus the tentacles nearest the apex of the hydranth 

 are the oldest and largest. The circles are indefinite, that is, the 

 tentacles of a whorl do not all arise from the same level, so that in the 

 advanced hydroid they have rather the appearance of being scattered 

 than arranged in circles. The tentacles when fully developed are 

 stout and filiform; and are capable of much extension and contraction. 

 Figures 51 to 55 of plate 35 illustrate various stages in the early develop- 

 ment of the hydranth; the youngest being about fifty hours and the 

 most matured some seventy hours old. Figure 53 (pi. 35) shows a 

 form in which the polyp arises from near the end of the hydrorhiza. 

 This is exceptional. A hydranth with the third circle of tentacles is 

 shown in figure 55 (pi. 35) ; the tentacles of the first whorl have become 

 considerably elongated. The hydrocaulus now becomes longer and 

 more slender and the hydranth assumes a fusiform body. 



The polyps that I reared from eggs were at the age of three days 

 like the hydranths of the adult colony found and figured by Professor 

 Brooks in the main features, except that they had not yet developed as 

 many tentacles. In his description he says: 'The upright stems of 

 the hydra, from 8 mm. to 12 mm. high, bore large terminal hydranths, 

 as well as smaller ones which were scattered irregularly along the stem 

 on short stalks. The long fusiform body of the hydranth carries 

 from eighteen to twenty thick, short, filiform tentacles, which are 

 arranged in three or more indefinite whorls. The medusa buds 



