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the digestive sac and following the walls of the main cavity. But 

 what is more striking and characteristic in Hydroid Polypi is the 

 circumstance that the main bulk of the body is constantly well 

 circumscribed, and its margin or surface provided with peculiar tenta- 

 cles, while the proboscis or mouth assumes various forms, projecting 

 sometimes in the shape of a movable tube, or in the form of a promi- 

 nent tubercle, with a central oral opening, encircled by a row, or 

 several rows, of peculiar tentacles or fringes. These oral appendages 

 may be compared morphologically to the fringed lobes arising from 

 the margin of the mouth in true Medusae, while the tentacles of the 

 margin and surface of the body bear the closest resemblance in posi- 

 tion and relations to the tentacles arising from the margin of the disc 

 in true Medusas. 



In these respects, therefore, the Hydroid Polypi are more closely 

 allied by structure to true Medusas than to Polypi, and their resem- 

 blance to Polypi it chiefly derived from the elongation of their ver- 

 ticle axis, the development of a peduncle of attachment, and the 

 formation of buds which remain attached to the main body, and give 

 it the appearance of a branching Polypi, while true Medusas are, from 

 the beginning of their independent development, free moving animals . 

 But it has been already ascertained in so many families, among Echi- 

 noderms as well as among Polypi, that there are types attached by 

 stems, and others which are entirely free, notwithstanding their closest 

 structural relations. That this fact can be no objection, but, on the 

 contrary, an argument in favor of the view that Hydroid Polypi, 

 with their medusine structure, should be considered as true Medusas, 

 provided with a stem, rather than a peculiar family among Polypi, 

 and even should there be among them forms which never produce 

 free Medusas, as soon as they present those peculiar combinations of 

 character which occur only among Medusas, we shall be inclined to 

 remove them from Polypi, and to place them among true Medusas. 

 The medusoid character of the crown in Tubularia is particularly 

 obvious, and it requires little familiarity with the different forms of 

 Medusas to feel satisfied that the inner prominent cylindrical bulb 

 which projects above the crown of large tentacles, and is provided on 

 its summit with another row of shorter tentacles, is truly analogous 

 to the central proboscis of many discoid Medusas, such as Gorgonia 

 and Sarsia, while the large tentacles around the central cup corres- 

 ponds to the marginal tentacles of Medusas. The stem itself is 

 analogous to the short stem by which the bulb of young Medusas is 



