REPORT ON THE HYDROIDA. H 



serrated margin of a leaf, and they are here usually adnate to one another, so as to form a 

 completely closed case (PI. XL fig. 4, and PI. XII. figs. 4 and 8). In other instances 

 they contract no adhesion by their edges, and the corbula is then open (PI. XI. fio-g. 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 corlntla is a simple 

 continuation of this peduncle, with the hydrothecse entirely suppressed. 



In determining the morphological significance of the corbula, the meaning of the 

 costse 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 costee as the greatly modified mesial nematophores 

 of the suppressed hydrothecae, 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- 

 caipal apparatus in other genera. 



The form of phylactocarp referrible to the type found in the Lytocarpxis {Aglaophenia) 

 myriophyllum of the European coasts, afibrds the means of clearing up this point in a 

 way which wiU 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 mt'^phology of the corbula, while another beautiful species, Acanthocladium 

 huxleyi, occurring in the Challenger collection (PI. X., and PI. 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 

 hydrothecse, 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 hydrothecse 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 hydrothecse may be recognised in nearly 

 its usual condition, while the mesial nematophore, though holding its normal position with 



' Hydroids of the Gulf Stream, pp. 44 and 46, pi. xxvi. figs. 1-8, and pis. xxvii, and xxviii. 



