88 LOWER INVERTEBRATES. 



There are the alert and active ones, ever on tlie watch, ready to seize the opportunity 

 as it conies, but missing it sometimes from too great impatience ; and tlie slow, steady 

 people, with very regular movements, not so quick perhaps, but as successful in the 

 long run ; and the dreamy, indolent characters, of which the Zygodactijla ,is one, 

 ahvays floating languidly about, and rarely suqirisod into any sudden or abrupt 

 exjircssion." 



Nothing is known of the develojiment of this form, as all attemjjts to raise the 

 eggs have 2)roved futile, and it is unknown whether it has a hydroid stage or not. 



Sub-Order IV. — Trachymedus.?;;. 

 Tiie Trachyinedusse are usually considered a distinct grou]) of Ilydroidea, espe- 

 cially characterized l)y having a direct development ; that is, they are jelly-fish, which, 



in general structure, are like the medusa? de- 

 Aelojjed from hjdroid colonies, but their eggs 

 develop directly into new medusie, and not into 

 hydroids. They have then no hydroid state. 

 They ai-e represented in our waters by a num- 

 ber of sjiecies, among them Trachynema digi- 

 tuk. The full-grown medusiE of this species are 

 from an inch to an inch and a half in length, 

 the walls are ver^' thin, and not much used for 

 locomotion ; the latter function being performed 

 princijially by the muscular velum which 2)ushes 

 itself outward with considerable force. The 

 proboscis is ■very large and has four liji-like ex- 

 pansions about the mouth ; the tentacles are 

 numerous, and four garnet-colored otocjsts are 

 present at equidistant jioints on the margin of 

 the bell. The ovaries develop from the upjier 

 parts of the radial canals, are cylindrical and 

 much elongated, hanging pendent in the bell, 

 and reaching nearly to the velum. 



Cunina, another genus of this grou]i, though 

 not so common as Trachynema, is not rare on our coasts, where it is rejiresented by two 

 s])ecies, C octonaria and C. discoides. An interesting fact in connection with these 

 forms is that they live on Turrito2)sis, a jelly-fish allied to the Stomatoca mentioned on 

 a preceding page. 



Sub-Order V. — Hydrocorallin^. 

 Among the manj- creatures that contribute to the building of a coral reef, are to be 

 counted certain hydroids. For many years there was no suspicion but that the Mille- 

 pore corals were built by true coral polyps. Professor Agassiz discovered their true 

 nature twenty-five years ago. Later, some other coral-making hydroids have been 

 thoroughly studied by Professor Moseley, of 0.\ford, late of the "Challenger" ex]>edi- 

 tion. They are very beautiful forms, and there are three kinds of zooids ; the ordinary 

 feeding forms, the reproductive kind, and the dactylozooids. The latter have no 

 mouth and gastric cavity, and possess only a tentacular function. 



Fig. 82. — Trachiinema tligitale. 



