HTDROID CORALS. 



57 



Hydractinia echinata (Fig. 37) forms masses (each- called a 

 hydrophyton) encrusting shells. 



In Clava the reproductive buds remain permanently aU 

 tached. It grows in pink masses on Fucoids, about half an 

 inch high, and is very common on our shores. It is repre- 

 sented in fresh water by Cordylophora lacustris Allman, 

 which lives at bached to rocks and plants in Europe and this 

 country. 



Here comes in the group of Hydroids represented by 

 Millepora and Stylaster, which were formerly considered to 

 be Anthozoan corals. By the researches of L. Agassiz in 

 1859, and H. M. Moseley 

 in 1876, Millepora, which 

 had been confounded 

 with the coral polyps, 

 has been proved to be a 

 Hydroid allied, as Agas- 

 siz stated, to Hydracti- 

 nia. Like that Hydroid, 

 it forms a calcareous 

 encrusting mass, but of 

 much greater extent, a 

 considerable proportion 

 of the coral in the Flori- 

 da reefs being formed 

 by the Millepora. Our 

 American species is Mil- 

 lepora alcicornis Linn., 

 while our description is taken from Moseley's account of 

 Millepora nodosa Esper. (Fig. 38). Its generic name is de- 

 rived from the numerous pores or calicles dotting its surface 

 and arranged in irregular circular groups, consisting of a 

 central caliele, or cup-like hollow, with from five to eight 

 smaller calicles arranged around it. The mass of the coral, 

 or hydrophyton, consists of fibres (canals or tubes) of lime, 

 forming a spongy mass, traversed in all directions by tor- 

 tuous spaces which " form regular branching systems with 

 main trunks, giving off numerous branches, from which 

 arise secondary branches, and from these again smaller 



Fig. 38.— Millepora nodosa. 

 zooid ; &, reproductive zooid ; 

 the same coiled lip in its cell ; t 



a. nutritive 

 €. lasso-cell ; d, 

 !, a third form. 



