THE PLUMULARIDjE. 



11 



almost unique in having the points of all the teeth curve abruptly inward. I know of but one 

 other species exhibiting this peculiarity, and that is Aglaophenia phyllocarpa Bale an Australian 

 form. 



The intrathecal ridge is a more or less extensively developed Ibid of chitine, which projects from 

 the inner surface of the anterior or posterior wall of the hydrotheca and extends forward or back- 

 ward in a horizontal or oblique direction toward the opposite side (fig. 22). The ridge may extend 

 almost around the hydrothecal cavity, thus partially dividing the latter into two portions. The 

 body of the hydranth seems to be capable of retracting itself into the chamber below the ridge, 

 thus securing mechanical protection from the latter. When the hydranth is expanded its walls 

 are indented by the ridge, which, if extensively developed, causes an abrupt bend in the axis of 



TYPICAL HYimOTHEC.K OF THE KLKl'THEROPLEA. 



Fig. 8 Pluimda i-ia hujriiij\>ra. Fig. 14. Alonotht'ca mfinjarctta. 



Fiy:- 9. Diplopteron grande. Fig. 15. Calviiiia mirabilis. 



Fig. 10. Antennopsin nt'gi-a. Fig. 16. fHt>l<>i'tt'i-nn fi>uiiit>iiinri. 



Fig. 11. rluinularia caulitheca. Fig. 17. J'ohijtluui ularia arinata. 



Fig. 12. A.ntennopai8 annalata. Fig. 18. I'linnulariaJ/liraulis (after Bale). 



Fig. 13. Monoxttrclin* ffuadridenR. Fig. W.^I'luinularia uoltlatfiiti (after I'.ale). 



the hydranth. Bale explains both the use and origin of the ridge as follows: "The use of the 

 intrathecal ridge is evidently to form a protective shield behind which the hydrauth can retire; 

 and if we consider its structure anil origin it will be sufficiently obvious that whether it springs 

 from the ba.ck or from the front of the hydrotheca its nature is essentially the same, and that it 

 originates from a fold or constriction of the hydrothecal wall, which is more or less bent upon 

 itself either towards or away from the hydrocladium, or in both directions alternately." 1 This 

 author then shows how, in his opinion, the ridge can always be regarded as the mechanical result 

 of the bending of the hydrotheca. This explanation appears to me to apply very well to the 

 anterior ridge in many species; but I am inclined to regard the posterior ridge, which is by far the 

 more common in American species, as strictly homologous with the septal ridges found so abun- 



1 The (iener:i of the I'liimuliiriid:!-, with observations on various Australian Hydroids, Melbourne, 1886, j>. 4. 



