Chap. IV. HYBOCODON PROLIFER. 243 



long tentacles, from the actinal side of which rises a long, tentaculated proboscis. 

 The medust^-buds arise upon the floor between the outer tentacles and the base 

 of the proboscis; they may become free medusfe, or remain sessile and wither. 

 The genera differ from one another, chiefly by the form and arrangement of the 

 tentacles of the proboscis and the structure of the medusae-buds. All the Tubu- 

 laridce have the same form, and constitute a very natural family. 



SECTION II. 



HYBOCODON PROLIFER AG. 



Proles hjdroidea. Adult. — We have never found Hybocodon elsewhere than in 

 the purest sea-water, in clear pools at low-water level. Notwithstanding frequent 

 explorations, it has not been discovered along our rocky shores where the tide 

 dashes backward and forward, and on this account we are inclined to believe that 

 it is, properly, a deep-water animal. The locality from which we are in the habit 

 of collecting it, is a ledge of rocks at Nahant, lying at a short distance from the 

 shore, and covered by ordinary tides; and it is only where the pools are protected 

 by a great roof of rocks that this Hydroid flourishes. It is easily detected by 

 its deep, orange-red color, and by its size, which is much greater than that of 

 any other littoral Tubularians. In fact, the only Tubularian with which it may 

 be compared, in size, is Thamnocnidia spectabilis (PI. XXII. Fig. 16), and that is 

 a brackish- water animal. It is seldom that more than three or four individuals 

 are found together, and we have not been able to ascertain whether they are 

 ever united by a common basis, as, from their position, the stems, to be secured, 

 had, in every case, to be cut away from the rocks on which they rested, without 

 any chance of tracing their relations to each other. However, it does not appear 

 that this Hydroid has the habit of branching so intricately as the genera Tubu- 

 laria, Thamnocnidia, and Parypha. The stem, which averages two inches in 

 length \V\. XXV. Fig. 1, a), is not thicker, at its base, than a common sewing 

 needle, but from this point it gradually enlarges toward the head, at the base of 

 which it has a diameter of one sixteenth of an inch. There is but one head 

 on each stem, to which it is joined by a constricted portion just below the 

 globose terminal expansion {Fig. 2, h). At times, the base of the head and the 

 end of the stem are very much distended {Fig. 3), and the constriction is totally 

 obliterated, so that it is impossible to tell where the stem terminates and the 

 head begins. This is a condition which we have noticed only toward the end 



