MICHIGAN ACADEMY OF SCIENCE. 99 



The difference in appearance between the male and female colonies — the 

 male sporosacs being often much elongated and of a yellowish tint, while the 

 female are rounded and rose-colored — caused Van Beneden to regard them 

 as two distinct species, which he described in a paper published in 1844 as 

 Hydractinia lactea and Hydractinia rosea. 



The dactylozooids are about the same size throughout their length. Their 

 distal extremity is surroimded by a circle of rudimentary tentacles from ten 

 to sixteen in number. There is a very small mouth in the center. These have 

 strong, muscular walls and are capable of coiling and uncoiling themselves 

 As their function is chiefly to protect the other members of the colony, they 

 possess an abundance of nematocysts (PI. 1, Fig. 1). 



The tentaculozooids are extremely slender, though often exceeding the 

 dactylozooids in length (PI. X, Fig. 5). They are capable of great extension, 

 and are characterized by Mr. Hincks as floating like long fishing lines through 

 the water. In preserved material, on the other hand, they are contracted to 

 such an extent as to render them extremely difficult to find. They are situated 

 near the outskirts of the colony, and are usually few in number as compared 

 with other members. The tip only is covered with nematocysts. No mouth is 

 present, and the gastral cavity is very small. 



These were regarded by Allman as abnormal dactylozooids on account of 

 their paucity. Colcutt, however, found them present in every colony of 

 Hi/dractinia echinata, and considered them as normally present. Mr. Hincks 

 reports them as constantly occurring. He also states that he distinguishes no 

 difference between his Hydractinia echinata, and Hydractinia polyclina of 

 Agassiz. I have found the Hydractinia polyclina of Woods Hole region to 

 correspond in every particular with Colcutt's Hydractinia echinata, but the 

 skeleton differs in minor details from that of Hydractinia echinata as described 

 by Mr. Hincks, to which reference is made under the discussion of the 

 skeleton. 



THE SKELETON. 



The skeleton is a chitinous structure which forms an irregular crust on 

 univalve shells, or other objects on which the colony is growing. The skeletal 

 structures penetrate the shell by dissolving the calcareous substances with an 

 acid, or erosive agent which the animal secretes. The chitin is then secreted 

 by the lower ectoderm of the coenosarc in thin layers (PI. 2, Fig. 4). Tliese 

 are so closely attached to the shell that the latter must be dissolved away with 

 dilute hydrochloric acid in order to obtain good specimens of skeleton. Pieces 

 of skelton can then be cut from the shell and thin sections made. 



The skeleton is overlaid by coenosarc, consisting of two layers of ectoderm, 

 enclosing between them a number of endodermal tubes which branch and 

 anastomose promiscuously. Tliese are connected at intervals with the canals 

 of the polyps, who.se ectoderm and endodei-m are continuous respectively with 



