TUBULARIAE 



93 



Order 2. HYDROCORALLINAE. 



Hydromedusans in which the polyps are colonial and have a calcified 

 perisarc of such thiekness that the colonies resemhle corals. In fact, 

 the animals were classified among the corals until Louis Agassiz in 

 1859 showed them to be hydromedusans allied to the Tubulariae. The 

 colony is incrusted on a rock or some other object and rises erect in the 

 form of a more or less arborescent, coral-like body in the water, being 

 composed of a network of tubes imbedded in a thick calcareous mass. 

 The tubes have the cellular structure characteristic of hydromedusans, 

 the calcareous groundwork being secreted by their ectoderm. Numerous 

 pores appear in the surface of the colony leading into cylindrical cham- 

 bers from the bottom of which two kinds of polyps may project into 

 the surrounding water; these are nutritive polyps or gastrozooids, with 

 mouth end often provided with tentacles, and the defensive polyps or 

 dactylozooids, without mouth and with batteries of nematocysts. The 

 gonosomes are usually sporosacs, but in a few forms they are medusae 

 and are produced in chambers which open to the outside through special 

 pores. 



The suborder contains 2 families and 15 genera, which are inhab- 

 itants of tropical seas. One species occurs on the Florida coast. 



Family MILLEPOEIDAE. 



Colony very varied in form, consist- 

 ing of a broad basal mass which is in- 

 crusted on the rock, and irregular, short 

 branches which rise from it into the 

 water; the nutritive polyps have each 4 

 or 5 short knobbed tentacles; the defen- 

 sive polyps are also provided with ten- 

 tacles ; the gonosome is a free medusa with 

 4 or 5 rudimentary tentacles: 1 genus. 



MiLLEPORA L. Each nutritive polyp 

 is surrounded by 5 to 6 long and very 

 contractile defensive polyps : 1 species on 

 the Florida coast and in the West Indies. 



M. alcicornis L. Pepper coral. On 

 the coast of Florida; has unusual sting- 

 ing powers. 



Fig. wo — A tubularian hydroid 

 polyp (Eudendrium) (Hertwig). 

 1, entoderm : 2, ectoderm ; 3, peri- 

 sarc ; 4, gastrovascular space ; 5, 

 mouth ; 6, hypostome. 



Order 3. TUBULARIAE. (Gymnoblastea; Anthomedusae.) 



Mostly colonial hj'dromedusans in which the hydranth is without a 

 protective cup (hydrotheca) (Fig. 150) and which produce either free 



