346 THE AMERICAN MONTHLY [Dec 



must be taken that there are living barnacles on the piece 

 of rock, as many are but empty shells. An interesting 

 slide is Obelia geniculata. It has double and alternate 

 generations. The polyp bears urn-like reproductive cap- 

 sules which discharge large numbers of medusiforra 

 zooids. Like miniature balloons they float suspended in 

 the water for a while, and then suddenly start into mo- 

 tion with a series of vigorous jerks. They may be con- 

 sidered as swimming polypites with the arms united by a 

 contractile web. They mature and disperse the genera- 

 tive elemtnts, and, having thus fulfilled their function, 

 perish. The ova, after fertilization, become ciliated em- 

 bryos, and when affixed rapidly grow into the plant-like 

 zoophytes we see. Sertularia pumila — another hydroid 

 zoophyte — is a very common species, though it makes a 

 very beautiful microscopic object. Almost every broad- 

 leaved seaweed has greater or lesser numbers of this zoo- 

 phyte growing on it. Coryne vaginata is one of the Atlie- 

 cata — that is, without any theca or calycla. The capitate 

 tentacles bear on the summit a globular head consisting 

 of a collection of thread cells, a vigorous battery of offen- 

 sive weapons. They occur in astonishing profusion, and 

 consist of minute sacs embedded in the flesh, filled with 

 fluid, which contains a long delicate thread capable of be- 

 in^ projected with considerable rapidity. Corydendrum 

 parasiticus is a creature similar to the last, but the tenta- 

 cles are not capitate. There is something singular about 

 the stems that supported the polypites ; they look as if 

 they acted as capsules and held ova. This is a foreign 

 species, and I cannot find any description of it. I be- 

 lieve Mr. ftinel told me it came from the Mediterranean, 

 Pennaria Carolina is also a foreign species. Some of the 

 polypites bear gonophores, the buds in which the repro- 

 ductive elements are formed. 



GrLYCERiNE MOUNTING MEDIA. — In using it is well to re- 

 member, as pointed out by Dr. Carpenter, that they large- 



