THE PLUMULAKID.E. 33 



his new genus CaUicarpa ( Plate XVII, fig. 0), which consisted of special branch"* which do not bear 

 hydrocladia, but are profoundly modi lied for the protection of the gonangia. The whole structure 

 somewhat resembles a spike of barley consisting of the central axis, from which arise branched 

 ribs arranged in verticils of three, each rib being divided into four branchlets and each terminal 

 branchlet being armed with a row of nematophores. The gouangia are borne in the axils of the 

 ribs. This is the most elaborate structure yet found among the Eleutheroplea for the protection 

 of the reproductive zooids. 



In the Statoplea there are only two genera, Xuditheva and Hnlwornaria, that are gymno- 

 carpal, all of the remainder being possessed of more or less specialized contrivances for the pro- 

 tection of the gouangia. The phylactocarps in this group may eventually be divided into three 

 classes: 



First, those which are, morphologically, modified hydrocladia. 



Second, those which are, morphologically, modified branches. 



Third, those which are, morphologically, appendages to hydrocladia. 



In the first group would be included the most conspicuous and the largest known form of 

 phylactocarp, the corbula, found in the genera Ayltiopheitia and Thecocarpiis. The corbtila is 

 strictly a highly modified hydrocladium, the proximal part bearing one or more hydrotheca' which 

 may be either normal, or more or less modified. It seems to be a general rule that when there is 

 only one hydrotheca between the corbtila and the stem, it is normal, as in most species of Aglao- 

 pheniu ( Plate XX, fig. 4), but where there are several hydrothecae between the corbula and the stem, 

 they are apt to be more or less modified, as in Thecocarpim. A better idea can be gained concerning 

 the appearance of these exquisite structures by an examination of the plates accompanying this 

 work than can possibly be conveyed by description, however elaborate. In general it may be said 

 that the corbula consists of an axial stem, or rachis, which is homologous with the hydrocaulus of 

 the ordinary hydrocladium, and a number of corbula leaves. This stem often shows indications of 

 divisions into regular internodes, each of which bears one of the corbula leaves or ribs. The leaves 

 often appear to be borne in pairs and are generally so described in technical language, especially in 

 systematic works. As a matter of fact, however, they are alternate, as are the hydrocladia them- 

 selves. Each leaf curves outward, upward, and then inward, the leaves on one side meeting those 

 of the other side above, the whole forming a pod-shaped receptacle, within which the gonangia 

 are protected. Ordinarily each leaf, in the mature corbula, is attached by its distal edge to the 

 one immediately in front, which it often overlaps slightly. When there is a definite space between 

 adjacent leaves, the corbula is called "open," and when the leaves are adherent along the edges 

 they form a "closed" corbula. On account of the overlapping of the leaves mistakes have arisen 

 in descriptive works regarding the presence of nematophores along the proximal edge of each 

 leaf. I have dissected a number of corbula'. of different species and have found, without exception, 

 that the distal edge of each leaf is armed with a regularly disposed row of large gonosomal nemato- 

 phores which often produce a very striking and beautiful external ornamentation. In addition to 

 this there is always, so far as the species dissected are concerned, a row of uematophores on the 

 proximal or inner edges of each leaf, the nematophores projecting into the cavity of the corbula. 

 This latter row is often concealed by the imbrication of the leaves, and thus it conies about that 

 corbula' are described as having leaves with a single row of uematophores, when, in fact, each leaf 

 has two rows, one of which is internal and concealed. 



As to the homology of the corbula leaves, they are, in the opinion of Allman, "th greatly 

 modified mesial nematophores of the suppressed hydrotheca-, complicated by the development on 

 them of secondary nematophores, and thrown alternately to the right and left in accordance with 

 their new protective function." 1 In my opinion, it is perhaps not possible to decide in every case 

 whether we have here a modified nematophore, or hydrotheca, or simply the modification of a 

 structure originally produced to protect what might be called an indefinite person, an individual 

 that might, under other circumstances, have eventually become either a sarcostyle or a hydranth. 

 In this view of the case attempts to homologize the leaves with nematophores or hydrauths are 

 unnecessary. 



'Reporl on the Hydroida dredged by H. M. S. Challenger during the years 1873-76, Pt. 1., Plumulari.hr, 

 1883, p. 11. 



12833 3 



