8 AMERICAN HYDROIDS. 



tares; and perhaps the growth-period idea, an old one by the way,' is as likely to prove correct as 

 any that has yet b?en suggested. 



Hydrocladia. Whatever may be the arrangement of the branches of the colony, whether the 

 main stem soon breaks up into a cluster or tuft of erect branches, or gives off regular branches 

 which again divide in a dendritic manner, or gives rise to regularly opposite or alternate branches, 

 the ultimate hydrotheca-bearing brauchlets or hydrocladia are, as a rule, regularly pinnate, rarely 

 opposite, in their arrangement. The departures from this rule are as follows: 



First. The hydrocladia may spring direct from a creeping rootstock, in which case the root- 

 stock may be regarded as a stem (Plate XVII, tig. 10). The hydrocladia would thus grow irregularly 

 from the upper side of the creeping rootstock, as in Antennella gracilis and one form of Plumularia 

 filicu iiiix. This unilateral arrangement is found again in the erect stein of Streptocanlus pulcherrimus 

 (Plate XXXIV, fig. 1), one of the species secured by the Challenger. In this case, however, the stem 

 itself is twisted so that the hydrocladia appear to be inserted along a spiral line ascending the 

 stem, the hydrocladia jointly forming a helix with the stem as an axis. In the genus Gattya the 

 hydrocladia spring directly from the creeping rootstock, but are borne on a jointed peduncle, and 

 sometimes bear what may be called secondary hydrocladia, springing from their sides. In the 

 genus Monontirchiix Alluian there is a unilateral arrangement of the hydrocladia in which the main 

 stem is dichotomously branched, and the hydrocladia always grow on the upper side of each branch. 



Second. The hydrocladia are arranged in verticils around the stem, the verticils being com- 

 posed of from three to eight or more hydrocladia. This arrangement is usually associated with a 

 peculiar structure of the stem by which the co?nosarr forms a number of anastomosing tubes and 

 is described as a canaliculated cu? nosarc. All of the species of the genus Antennularia as here 

 described are characterized by this verticillate arrangement of the hydrocladia, at least in the 

 adult colonies (Plate IX, fig. 3). 



Third. The hydrocladia are scattered irregularly on all sides of the stem. This unsymmetrical 

 arrangement is found only in the genus Antainopsis Alluian. It sometimes happens that the 

 proximal hydrocladia will be opposite and the distal hydrocladia scattered, as in young specimens 

 of Antcnnnlaria ; or the proximal hydrocladia may be alternate and the distal hydrocladia scat- 

 tered, as in Anlennopsis annulata (Plate XII, fig. 7). In almost all Plumularidie the hydrocladia are 

 divided into regular internodes. There are occasional exceptions, such as SeMssotricha dichotoma 

 and IHploptvron grandc, in which the nodes are either absent or unrecognizable. Throughout the 

 Statoplea each node is hydrothecate, but there are many eleutheroplean forms in which there are 

 one or more intermediate iuternodes. In the latter case the hydrocladiate interuodes are usually 

 the longer. ' The nodal joints are ordinarily nearly at right angles with the axis of the hydro- 

 cladium, but in the " Catharina group r of the genus Plumularia the uodes are alternately at right 

 and oblique angles with the axis. 



The hydrocladia are ordinarily uubranched, the main exception being the genus Schizotricha 

 (Plate XV, fig. 5) among the Eleutheroplea and Nuditheca dalli among the Statoplea. 



In a large number of species belonging to various genera the axial cavities of the hydrocladial 

 internodes are partially divided by internal thickenings of the perisarc, which form raised circular 

 ridges on the internal surfaces. These ridges, known as septal ridges, are found more frequently 

 in the Statoplea, especially the genera Clodocarpm, Thecocarpios, aud Aglaophenopsis, than in 

 the Eleutheroplea, where they are seldom seen, except in the " Lagenifera group ' of Plumularia 

 and in one or two species of Antennularia. The office performed by the septal ridges in the economy 

 of the colony is probably that of adding strength to the hydrocladia, although it is not unlikely 

 that there is some other utility involved. It is often difficult to distinguish between septal ridges 

 and nodes, and it is more than likely that the former have been mistaken for the latter, and 

 descriptions thereby rendered inexact. The distinction between them is not difficult to understand 

 it' one bears in mind that the nodes are always indicated when there are external annular 

 depressions in the perisarc when examined in profile, while the septal ridges are entirely internal 

 and make no depression in the profile of the internode. 



It sometimes happens that a normal liydrocladium loses its hydrotheca>, nematophores, and 

 nodes ; grows rapidly, and is transformed into a stolon or rootstock, from which a new colony 



1 George Jobuston, A History of hritish Zoophytes, Londou, 1817, j>. 100. 



