88 LOWER INVERTEBRATES. 



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

 as it comes, but missing it sometimes from too great impatience ; and the 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 Zygodactyla is one, 

 always floating languidly about, and rarely surprised into any sudden or abrupt 

 expression." 



Nothing is known of the development of this form, as all attempts to raise the 

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



Sub-Okdbr IV. — Teachymedtjs^. 



The Trachymedusse are usually considered a distinct group of Hydroidea, espe- 

 cially characterized by having a direct development ; that is, they are jelly-fish, whicli, 



in general structure, are like the medusae de- 

 A'eloped from hydroid colonies, but their eggs 

 develop directly into new medusae, and not into 

 hydroids. They have then no hydroid state. 

 They are represented in our waters by a num- 

 ber of species, among them Trachynema digi- 

 tate. The full-grown medusae of this species are 

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

 the walls are very thin, and not much used for 

 locomotion ; the latter function being performed 

 principally by the muscular velum which pushes 

 itself outward with considerable force. The 

 proboscis is very large and has four lip-like ex- 

 pansions about the mouth ; the tentacles are 

 numerous, and four garnet-colored otocysts are 

 present at equidistant points on the margin of 

 the bell. The ovaries develoiJ from the upjjer 

 parts of the radial canals, are cj'lindrical and 

 much elongated, hanging pendent in the bell, 

 and reaching nearly to the velum. 



Cunina, another genus of this group, though 

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

 species, C. octonaria and C. discoides. An interesting fact in connection with these 

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

 a preceding page. 



SuB-OeDER V. — HYDEOCOEALLIKiE. 



Among the many 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 Oxford, late of the "Challenger" expedi- 

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

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

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



Fig. 82. — Trachyiiema digitate. 



