38 THE VOYAGE OF H.M.S. CHALLENGER. 



canal; just as in the common octoradial Hydromedusai (e.g., Trachynema, Rhopalonema). 

 This regular corona of eight simple radial tentacles is permanent in the simplest and oldest 

 form of the Disconectse (Discalia, PI. XLIX. figs. 1-4). In all other genera of this 

 order the number of tentacles is rapidly increased, either by interpolation of eight inter- 

 radial secondary tentacles between the eight perradial primary ones, or by budding of 

 new secondary tentacles on both sides of the base of the primary ones, so that there 

 arise eight bunches of tentacles (Disconalia, PL L. ; Porpalia, PL XLVIIL). In the 

 larger Porpitidas their number is afterwards so multiplied, that the margin is armed with 

 a rich corona of many hundreds, or even thousands of tentacles (Porpema and Porpita, 

 Pis. XLVII. and XL V.). They are here densely crowded, and arranged in concentric 

 girdles (sometimes sis to nine or more) ; the uppermost (or proximal) girdle usually bears 

 the smallest, and the lowermost (or distal) the largest filaments. Their number is much 

 smaller in the Velellida?, where they form only a single submarginal series in Rataria 

 (PL XLIV.) and Velella, a double (or rarely multiple) series in Armenista (PL XLIIL). 

 Also in this family the original number seems to be eight, and in some smaller forms are 

 found sixteen ; but in consecmence of the bilateral development of the umbrella, their 

 number and arrangement is often modified, bilateral, or irregular. In the young larval 

 forms (Ratarula) often two primray tentacles, situated at the opposite poles of the major 

 axis (or sagittal diameter) of the elliptic disc, appear earlier than the others ; this 

 heterochronism is certaiuly kenogenetic. 



Structure of the Tentacles. — The tentacles of the Disconectas are very different from 

 those of all other Siphonophora? ; they are relatively short and thick, rather rigid, and 

 their movements are sluggish, as in most TrachoniedusEe. In general they are far 

 less extensile and contractile, and do not exhibit that peculiar development and movement 

 which are obvious in most of the Siphonanthas, and are similar to that of the Anthomedusas. 

 The body of each tentacle in all Disconectas is a hollow cylinder with a very strong 

 muscular wall and a narrow canal, closed at the distal end and opening at the proximal 

 end into the annular canal of the margin, or the marginal zone of the canal network. 

 The wall is composed, as usual, of the following five strata, enumerating them from with- 

 out inwards: — (l) An exodermal epithelium, armed with cnidoblasts, often vibratile in 

 some parts ; (2) a strong layer of longitudinal muscles ; (3) a thiu, but firm and elastic 

 structureless supporting lamella ; (4) a thin layer of ring-muscles ; (5) a vibratile 

 entodermal epithelium, lining the central canal, composed of very large vacuolate 

 entoderm cells similar to the axial cells in the tentacles of many Trachornedusse. 



The armature of the tentacles with cnidoblasts exhibits characteristic differences in 

 the families of Disconcctse. Discalia (PL XLIX. figs. 1-4), as the simplest form of all, 

 and likewise probably the youngest larval stages of all Porpitidae, possess eight simple 

 tentacles, which bear a single cnidosphere (or a spherical knob composed of cnidocysts) 

 at their distal end (PL L. fig. 9). 



