REPORT ON THE HYDROIDA. 11 
serrated margin of a leaf, and they are here usually adnate to one another, so as to form a 
completely closed case (Pl. XI. fig. 4, and Pl. XII. figs. 4 and 8). In other instances 
they contract no adhesion by their edges, and the corbula is then open (PI. XI. figs. 5 
and 9). 
That the corbula is a modified hydrocladium there can be no doubt. It takes exactly 
the place of a hydrocladium, while its proximal end is in the form of a short peduncle, 
which holds it to the stem, and which still retains the normal condition of the ramulus, 
and carries a scarcely modified hydrotheca. The floor or rachis of the corbula is a simple 
continuation of this peduncle, with the hydrothecze entirely suppressed. 
In determining the morphological significance of the corbula, the meaning of the 
cost or leaflet-like ribs becomes an element of primary importance. Now the key to 
this will be found in certain other forms of the protective apparatus, and I believe we 
shall be justified in regarding the coste as the greatly modified mesial nematophores 
of the suppressed hydrothecee, complicated by the development on them of secondary 
nematophores, and thrown alternately to the right and left in accordance with their new 
protective function. This will become apparent after an examination of the phylacto- 
carpal apparatus in other genera. 
The form of phylactocarp referrible to the type found in the Lytocarpus (Aglaophenia) 
myriophyllum of the European coasts, affords the means of clearing up this point in a 
way which will scarcely admit of doubt. Two beautiful species, Lytocarpus (Aglaophenia) 
distans, and Lytocarpus (Aglaophenia) bispinosa, obtained during the exploration of the 
Gulf Stream by the United States Survey,’ are especially significant in the light they 
throw on the morphology of the corbula, while another beautiful species, Acanthocladium 
hualeyi, occurring in the Challenger collection (Pl. X., and Pl. XX. fig. 1), is scarcely 
less instructive. 
In all these, as well as in Lytocarpus myriophyllum, the phylactocarp is, as in the true 
corbula, an obviously modified hydrocladium. After retaining for some distance from its 
point of origin the normal character of the hydrocladium, and supporting one or more 
hydrothec, each with its usual mesial and lateral nematophores, it is continued in an 
altered form, and develops a double series of long ribs, which carry numerous small 
nematophores along one or both edges, remain quite distinct from one another, and form 
the walls of an open basket or cage (Pl. XX. fig. 1), along whose floor the gonangia 
are distributed from the proximal to the distal end. Now, in this continuation of the 
hydrocladium the hydrothec are not, as in the true corbula, suppressed. We find, 
on the contrary, that every rib carries a hydrotheca at its base, the rib with its basal 
hydrotheca being raised on a peduncle from the rachis or floor of the cage. The pair of 
lateral nematophores belonging to each of these hydrothecee may be recognised in nearly 
its usual condition, while the mesial nematophore, though holding its normal position with 
1 Hydroids of the Gulf Stream, pp. 44 and 46, pl. xxvi. figs. 1-8, and pls, xxvii, and xxviii, 
