ON THE STRUCTURE OF HYDRAOTINIA ECHINATA. 89 



The Polyps. 



1. The Gasterozooids. — The external appearance of these 

 polyps has heen frequently described and figured ; they are 

 cylindrical in shape and very contractile, and may attain to a 

 height of about a quarter of an inch. They are provided with 

 a conical hypostomC; terminating in a mouth, and having 

 round its base two closely approximated series of tentacles, 

 which increase in number with the age of the polyp, and vary 

 from ten to thirty in different colonies. These tentacles are 

 very contractile, and are provided with numerous nematocysts. 

 The polyps are capable of everting their hypostomes. The 

 outermost cell-layer of a Gasterozooid is a layer of ectoderm, 

 which is continuous at the base of the polyp with the upper 

 ectodermal layer of the basal coinosarc ; the alimentary canal 

 is lined by a single endoderra layer, which forms a wide 

 cylindrical tube, connected at its base with the plexus of en- 

 dodermal tubes which permeates the basal coenosarc (PI. 1, 

 fig. 1, c). Between the ectoderm and endoderm is the well- 

 marked mesoglcea. 



2, The Blastostyles. — These are specially modified re- 

 productive zooids, giving rise to " sporosacs.^^ The blasto- 

 styles are smaller than, and differ considerably in appearance 

 from the Gasterozooids ; they have a small mouth at the apex 

 of a conical " head," and at a short distance from this terminal 

 mouth there are two or more closely approximated circles of 

 tentacles. These tentacles vary in number from ten to thirty ; 

 they are rudimentary knob-like structures, containing prolon- 

 gations of the endoderm and mesoglcea of the polyp, and their 

 ectoderm is crowded with nematocysts (PI. 1, fig. 6). 



Below the tentacles the body narrows gradually, becoming 

 externally constricted into a narrow " neck ; '^ then it widens 

 considerably into a globular dilatation, from the walls of which 

 arise the round or oval sporosacs. The sporosacs are out^ 

 growths of the walls of the polyp, all three layers of the polyp 

 being involved in their formation : generative cells arise in 

 the body of the blastostyle and migrate into the sporosacs. 



