ACAULIS. 151 



small portion of the upper part of the chymiferous tubes (Figs. 227, 

 228), but with advancing age extend farther down (Figs. 230, 231), and 

 in the adult they reach the circular tube. The genital organs remind 

 us, in their mode of growth, of what we find in Melicertum and Stau- 

 rophora. The outline of the bell is but little changed from the earliest 

 stages to the more advanced ; it simply grows Fig . 2 3i. 



somewhat more globular. The sensitive bulb 

 as well as the ovaries are slightly yellowish. 



McCrady describes the Hydroid of this 

 Medusa as a free floating community ; I 

 greatly incline to the opinion of Professor 

 Allman, that we have in these free Hydroids 

 nothing but the detached head of some Tubu- 

 larian ; certainly the figures given by Mc- 

 Crady of the Hydroid of Nemopsis, and by 

 Stimpson of Acaulis, remind us very forcibly 

 of detached heads of Tubularians. The heads 

 of our Pennaria (Globiceps tiarella Ayres) 

 frequently drop off, and nothing is more common than to see, at the 

 time of breeding, several of these heads, covered with Medusae, floating 

 about in the jars where the Pennarige are kept, and to have the Me- 

 dusae buds come to maturity while the head is thus detached, and 

 would readily be mistaken for something like a free Hydroid. During 

 four successive summers I have hunted in vain in the hope of finding 

 one of these free Hydroids among the innumerable small Medusae 

 which must have just separated from the Hydrarium, which makes it 

 probable that the Hydrarium is fixed, and not floating. 



Vineyard Sound (L. Agassiz) ; Buzzard's Bay (A. Agassiz) ; Charles- 

 ton Harbor (McCrady). 



Cat. No. 44, Nantucket, Mass., June, 1849, L. Agassiz. Medusa. 



Cat. No. 272, Naushon, Mass., Sept. 1861, A. Agassiz. Medusa. 



ACAULIS STIMPS. 



Acaulis STIMPS. Mar. Inv. Grand Manan, p. 10. 1853. 

 Acaulis AGASS. Cont. Nat. Hist. U. S., IV. p. 345. 1862. 



Acaulis primarius 



Acaulis primarius STIMPS. Mar. Inv. Grand Manan, p. 10, PI. 1, Fig. 1. 

 Acaulis primarius AGASS. Cont. Nat. Hist. U. S., IV. p. 345. 18G2. 



Grand Manan (W. Stimpson). 



Cat. No. 162, Grand Manan, W. Stimpson. Hydromedusarium. 



Fig. 231. Magnified view of the genital organs, the actinostome, and the oral tentacles. 



