98 THE AMERICAN MONTHLY [April, 



reproduce sexually or found other colonies like the one from which 

 they sprang. These meduste are broad and flat, not bell-shaped, 

 and have no veil, but they have marginal tentacles, radial vessels 

 manubrium, and mouth. (The development of the obelia egg 

 is described by Haddon, Practical Embryology, p. 49.) 



Sertularia pumii.a.* 

 Here, as in Obelia, we have a colony of extremely minute 

 zooids, the colony itself having the appearance of a plant, 

 whence its name of sea-moss has been applied to it. The 

 colonies are generally found abundantly on the green rock-weeds 

 {JFucus)^ so very numerous on the rocky shores of the New- 

 England and Middle vStates. The colony as a whole is short 

 and usually closely applied to the rock-weed. It consists of a 

 central stem and lateral branches which arise from it and slant 

 away from the base of the colony. The stem appears to be 

 notched ; these notches when closely examined reveal the 

 chitinous hydrothecae, which lodge the extremely small 

 feeding zooids. The cups are sessile on the stem, not stalked 

 as in Obelia, and they are opposite each other (they are 

 alternate in Hydrallviania) ^ and each one has a little cover, 

 operctilum,, to close the end of the hydrotheca when the zooid 

 is retracted. The feeding zooids are connected by the fleshy 

 main stem, so that, as in all hydroid colonies, the combined product 

 of their digestive processes can form a sort of blood, and circu- 

 late throughout the entire colony and supply every member. 

 The colony includes, besides the feeding zooids, fewer gono- 

 zoids ; these are contained in larger capsules of chitine ; they 

 present a stem which produces medusae by budding. ThemedusEe, 

 in the case of S. pumila^ however, and in many of its allies, have 

 no mouth and never become free, but they produce eggs or 

 spermatozoa, and set them free, after which they are of no 

 further use to the colony. 



NaNOMIA CARA.f 



In all the hydroids mentioned up to this point I think any one 



*BiBi.ioG. — Agassiz, Seaside Stud., p. 66. 



Packard, (S. abietina) Zool., p. 6i. 



S. argentea, Riv. Nat. Hist., p. 86. 



Explanation of the Figure of Sert. pumila. 

 St., common stem of one of the ultimate suudivisions. 



Hy, single feeding zooid, and shows also its hydrotheca ; the operculum shows in the one 

 above. 

 Gon, a gonozooid stem ; the se.xual buds, and the large gonotheca covering them. 

 tBiaLIOG. — Agassiz, Seaside Studies, p. 76. 



Lankester E. I'.ritt, ix, p. 564. 



Fewkes, Bull. Mus. Comp. Zool., xiii, p. 213. 



Huxley, Oceanic Hydrozoa. 



Explanation of Figure of Nanomia cara. 

 From Fewkes with slight changes. 

 Fl, the float at the closed end of the main stem. 



St, the main stem, at the lower end of which is the parent zooid of the colony. 

 Nc, nectocalyx or sterile bell for swimming. 

 Hy, one of the feeding zooids. 

 Ten, portion of one of the tentacle?. 

 Gon, gonozooids. 

 Sc, hydro-pKyllum or scale covering the zooids. 



