10 



AMERICAN HYDROIDS. 



There are certain special features of the hydranth in this group, however, that are worthy of 

 presentation, one of which is what may be called the protractor of the hydranth. This is a band 

 of tissue, probably ectoclermal, that originates from a point on the hydranth body about halfway 

 between the tentacles and base on the abcauline side and passes obliquely upward and outward till it 

 meets the inner surface of the abcauline wall of the hydrotheca, where it is firmly attached. (See 

 figs. 10, 11, 12 j)). It is obvious that a contraction of this band would aid in the protraction of the 

 hydranth, and also in the initial stages of its retraction. The best examples of this structure 

 that I have seen are in certain species of Sc/'t>/?</>'<-/ltt, as S. in<i<j,U<ini<-u (tig. 10), S. levmseni 

 (tig. 11), and S. ii't/nxtin/i<i (fig. 12). Hartlaub, in discussing this structure, 1 .says that it is 

 formed before the differentiation of the tentacles and is evident upon the first withdrawal of the 

 hydranth, and that in many species this band of attachment ("Haftzipfel ") makes a mark as if it 

 were a septum instead of a band, which divides the dorsal (abcauline) part of the hydrotheca into 

 two chambers. That portion of the hydranth body wall to which the, band is attached seems to 



p p - 



P-- 



12 



10 



11 



HF.TKACTED HYDRANTIIS, SHOWING PROTRACTORS. 



Fig. 10. S< rt//lai-/lhi nni'ji U'niirn. j>, j>rntr;irh>r: r. retractor; .-. septum. 

 Fig. 11. .">'/ riiihn -i-lln I' mnsent (lettering the suincO- 

 Fig. 11!. .S .rlulurclla mri/nttoiiia (lettering the simie). 



be stretched outward into a hernia-like protuberance, which contains a sack-like divarication of 

 the hydranth body cavity. To this latter Hartlaub has given the name " Blindsack." (Fig. 13, 1> .v.) 



In the material at my disposal it is difficult to ascertain the relationships of the various parts 

 in the proximal end of the tiydranth and bottom portion of the hydrotheca. In many species of 

 S, i'fiilin'1-llii the septum at the bottom of the hydrotheca extends more than halfway across from 

 the adeauline to the abcauline side, leaving an aperture that is unsymmetrically placed, being on 

 the abcauline portion of the bottom of the hydrotheca. The connection between the hydranth 

 and the sarcode of the stem passes through this aperture. (Fig. 14 \ c.) 



It appears that a large portion of the bottom of the hydranth is permanently attached to the 

 septum, and this part of the hydranth corresponds to the foot of the hydra. At times this 

 portion of the foot seems to be muscle-like in function and to work in opposition to the protractor 

 mentioned above. It thus serves functionally as a retractor muscle, and secures its point of resist- 

 ance on the upper side or face of the septum. This condition of affairs seems to be common in 

 those species that have a well-developed protractor on the abcauline side, and this fact would seem 

 to support the view that it works functionally in opposition to the latter. 



1 Revision der Sertularella-Arten, 1900, pp. 10, 11. 



