456 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 



ment. Within the last few years, however, a number of observers 

 have discovered this phenomenon in the developmental stages of vari- 

 ous' forms. 



ATTACHMENT. 



Under favorable conditions when the larva is about fifty hours old 

 it reaches that stage of development at which attachment takes place. 

 In preparation for this process the planula settles to the bottom, 

 loses its cilia, and consequently its movements cease. The manner 

 of attachment in Turritopsis like that in Stomotoca differs from that 

 usually described in hydroid development. Instead of settling down 

 on the anterior end of the planula according to the method which 

 occurs in Eudendrium, and which has been regarded as typical and 

 used in descriptions of the embryology of the hydromedusae in text- 

 books, the planula becomes attached on its side by nearly its whole 

 length and is transformed into a root. The hydrant h instead of 

 growing up from the posterior end of the planula as in forms which 

 attach themselves by the anterior end, develops from a bud that is 

 given off from the root, usually at about the middle. 



Professor Brooks observed the fact that the planula is transformed 

 into a root in Turritopsis, Eutima, and Hydractinia, and gives a brief 

 account of the same in his paper on "The life history of Eutima" 

 (Brooks, '84). Metschnikoff ('86) describes and figures for Mitro- 

 coma the fact that the larva becomes attached by its side and is almost 

 wholly employed in the formation of the hydrorhiza, while the first 

 hydranth grows out of it by a kind of budding. 



In general the attachment of the planula is similar in Turritopsis 

 to the method which is followed by Stomotoca, but the former does 

 not commonly produce secondary hydrorhiza. In Stomotoca at about 

 the time the hydranth bud appears, or even before, the root branches, 

 giving rise usually to one or two secondary roots. In Turritopsis 

 this branching rarely takes place, at least during the first few days 

 of the development of the hydranth. 



Professor Brooks describes and figures in the planula of Eutima an 

 ectodermal adhesive gland. It occurs after the endoderm and the 

 digestive cavity are formed, and before the appearance of the mouth 

 as an ectodermal imagination at the small end of the planula. In 

 Turritopsis no such special organ of attachment is found. The larva 



