I. HYDROZOA: HYDRARIA, CAMPANULARL 217 



Gonads on the radial canals; tentacular statocysts; develop directly to medusae 

 (rig. 180, A). (4) NARCOMEDUS^;. Gonads on the manubrium or gastral 

 pouches; tentacular statocysts; no polypoid stage (fig. 180, B.) (5) SIPHONO- 

 PHORA. Polymorphic, free-swimming colonies of Anthomedusas, no polyp 

 generation. 



As there are medusae without polyp stages and polyps without medusae, 

 a natural system must take into account both these features. When the life 

 histories are traced it is seen that the Anthomedusa; and the Tubulariae are 

 connected by an alternation of generations, as in Leptomedusas and Campanu- 

 lariaa. There are three groups Trachymedusae, Narcomeduste, and Sipho- 

 nophora without a hydroid stage, and two in which the polyp plays the chief 

 role, the medusa being rudimentary _in the Hydrocorallinae, lacking in the Hy- 

 draria. The hydroid polyps are usually but a few millimeters or fractions of 

 a millimeter in size, but the huge Monocaulis iwperator, of the deep seas, two 

 yards in length, forms an exception. The colonies are usually only a few inches 

 in extent. The medusae have bells varying between a millimeter and a few 

 inches in diameter (sEquoria forskalea sixteen inches). 



Order I. Hydraria. 



Until recently only the cosmopolitan species of Hydra were known. During 

 most of the year they reproduce by budding (fig. 93), only occasionally develop- 

 ing gonads (fig. 169). The eggs remain in connexion with the mother during 

 segmentation, and later form an embryonal shell. In this 'encysted stage' they 

 can be distributed by wind or water birds. These animals formed the basis 

 of the celebrated researches of Trembley on regeneration. He showed that 

 small portions w 7 hich included both body layers could regenerate the whole 

 animal. His experiments upon turning the animals inside out have not been 

 fully confirmed; for in such cases the layers resume their normal positions. 

 Hydra grisea* (fusca), brown; H. viridis,* green, from the presence of symbiotic 

 algae. Protohydra rydcri* without tentacles. 



Order II. Hydrocorallinae. 



Exclusively marine, forming colonies of thousands of polyps whose cal- 

 careous skeletons so resemble true corals that they were associated with them 

 until the animals were studied. Millepora alcicornis* (fig. 174), stag-horn coral, 

 in Florida. The rosy Stylasters in tropical seas. 



Order III. Tubulariae =Anthomedusae (Gymnoblastea). 



As a rule these colonial forms with perisarc but without hydrotheca produce 

 anthomedusse, but there are forms like Clava* and Hydractinia* which have 

 sporosacs. Indeed, Coryuwrplia* and Monocaulis* differ only by medusa 1 in 

 the former and sporosacs in the latter. The medusae have the gonads on the 

 manubrium, lack statoliths, and usually have a high-arched umbrella, and 

 frequently eye spots. In the forms with alternation of generations different 

 names are applied to the hydroid and medusan stages. 



Amon'T hydroids are Pennaria,* Syncoryne,* Endendri,* Tubularia,* among 

 medusae Sarsia,* Turritopsis* Margclis* Nemopsis.* 



Order IV. Campanulariae = Leptomedusae (Calyptoblastea). 



These forms differ from the last in that they are always colonial and possess 

 hydrothecaa, the medusae always being flattened Leptomedusae (p. 216). A 

 peculiarity is the existence of gonothecas, closed perisarcal envelopes, inside 



