4 AMERICAN HYDROIDS. 



ouly one or two main branches .springing from tbe proximal part of the main stem, a very com- 

 mon arrangement in the genera Aglaophenia and Cladocarpus. Others often give off a branch or 

 two quite bigb up on the main stem, as is the case in Aglaophenia rigida. Some are very straggling 

 in habit, resembling vines more than trees; for example, the northern variety of Monoxtarhas 

 dkhotoma. Of fiabellate forms there are many, especially in the statopleau section of the family, 

 as in the genera Lytocarpus and Aglaophenopsis. 



In size some phunularians may well be regarded as giants among the hydroids, a height of 2 

 feet being attained by full-grown colonies of a number of species such as I'litinnltu-ia ih-tulritica, 

 Thecocarpus benedicti, Gladocarpnx ijnuulis, and Cladocarpus paraditsea, while a specimen of 

 Thecocarpus in my possession from the bay of Naples attains a height of over 3 feet. Semper 

 speaks of a plumulariau which is "man high." This is probably the largest reported hydroid, with 

 the exception of Monocaulux Imperator Allman, a titanic uoubrauched gyrunoblastic form secured 

 by the Challenger, which attains a height of nearly 8 feet. 



But the Plumularidu', although averaging larger than any other group of hydroids, are not 

 all giants by any means, several species normally attaining a height of only about one-fourth of 

 an inch; for example, 1'lumnlaria filicaulis, Monotheca inar</nrctta, Guili/a /twuis, and Aglaophenia 

 perpusilla. Whatever their size, however, these forms are always exquisitely graceful and beau- 

 tiful, delighting not only the naturalist, but even the most unscientific observer who looks upon 

 the plumulariau as a very pretty form of ''seaweed." 



Stem-. This may be nonfascicled or fascicled. In the former case it consists of a single tube, 

 which is ordinarily divided into regular iuternodes. It may be straight, or there may be sudden 

 bends at the nodes, producing the geiiiculate stem. The branches, when present, are in most 

 cases identical in structure with the stem. The proximal portion of the colony is usually devoid 

 of hydrocladia and the stem is here straight, without outgrowths of any kind except nema- 

 tophores. In a number of the smaller species of Aglaophenia which grow from a creeping root- 

 stock there are peculiar twists in the stem below the pinnate portion, as if the stem had been 

 pinched and twisted in one or more places. Some authors regard these as nodal marks, but I can 

 see nothing to indicate this. 



Those portions of the stem and its branches which bear hydrocladia are characterized by the 

 fact that each internode bears a stout brace for the support of a hydrocladiuui, and usually two 

 or more nematophores. Species with nonfascicled stems do not usually attain the great size 

 reached by those with fascicled stems. Perhaps the largest simple-stemmed species among 

 American forms are Cladoi-urpux fej-Ulx Verrill, which attains a height of 9 or 10 inches, and 

 Halicornaria speciosa Allman, a heavy plumose form with a thick stem reaching a height of 

 12 inches. 



A very curious modification of the nonfascicled stem is characteristic of the genus Antennu- 

 laria, and was first noticed by Professor Allman (tig. 1). The stem consists of a single strong 

 tube of perisarc enclosing a series of cu-nosarcal tubes, each surrounded by ectoderm and 

 eudoderm. These ccrnosarcal canals, as they are termed, have a course which is in general 

 parallel to the axis of the stem. The several tubes send off frequent branches or offsets, which 

 form lateral connections with adjacent tubes, the whole forming a loose network of anasto- 

 mosing tubes. Where branches or hydrocladia are given off from the main stem, one or two of 

 these c<enosarcal canals will be diverted into the branch or hydrocladuim. The main cavity or 

 lumen of the stem is entirely empty so far as structural tissues are concerned, and is presumably 

 filled with sea water during the life of the colony. It is not homologous with the central cavity 

 of the ordinary nonfascicled stem, the whole structure resembling a polysiphonic or fascicled 

 stem in which only a single outer layer of tubes remains, and in which the perisarc of the indi- 

 vidual tubes is modified to form a single great tube inclosing the whole. This stem is divided 

 into distinct internodes, each of which bears a circlet or whorl of hydrocladia. The canalicu- 

 lated cu'iiosarr described above is found only in the genus Antennularia, and is a structure 

 unique among the Ilydroida. 



The fascicled stem consists of a tube, which, from the fact of its bearing the hydrocladia, I 

 will call the hydrocladiate tube, supported by a varying number of accessory tubes (fig. 2). The 

 former, or hydrocladiate tube, can always be recognized by the fact that it bears either hydro- 

 cladia or the stumps of obliterated or metamorphosed hydrouladia, and is distinctly divided into 



