ACAULIS. 151 



small portion of the upper part of the chyiaiferous tuhes (Figs. 227, 

 22S), hut with ndvanciug age extend farther down (Figs. 230, 231), and 

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

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

 rophora. The outline of the hell is hut little changed from the earliest 

 stages to the more advanced ; it simply grows Fi„ 231. 



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 

 Stimjison of Acaulis, remind us very forcibly 

 of detached heads of Tubularians. The heads 

 of our Pennaria (Globicejis tiarella Ayres) 

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

 time of breeding, several of these heads, covered with Medusa?, floating 

 about in the jars where the Pennarise 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 

 prol)able 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. Grantl Manan, p. 10. 185.3. 

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



Acaulis primarius Stimps. 



Acaulis primarius Stimps. Mar. Inv. Grand ^lanan, p. 10, PI. 1, Fig. 1. 

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



Grand Manan (W. Stimpson). 



Cat. No. 102, Grand Manan, W. Stimpson. Hydromedusarium. 



Fig. 231. M.agnilioil viow of llio gcnlt.il organs, tlii' arlinostonu', and tlic oral tentacles. 



