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



tertiary septa into chambers of the third (sometimes even of the fourth) order (compare 

 Olfers, 79, fig. 1 ; Leuckart, 81, fig. 2 ; L. Agassiz, 36, fig. 1, &c). Possibly the number of 

 these chambers corresponds to that of the cormidia, which arise in metameric succession 

 from the opposite ventral side of the trunk ; it is different in the various species. The 

 Physalidse are able to compress the float and the crest in a very variable manner, and to 

 change their form in a most extraordinary degree. The external form of these hydro- 

 static organs, therefore, is of little value for the distinction of species. 



Cormidia. — The numerous groups of polymorphous persons and organs which 

 compose the corm of the Physalidse are usually loose, and represent, especially in the 

 larger species, a clustered mass of crowded parts, which seem to be aggregated without 

 any regular order. A comparative examination, however, of the younger stages and 

 larvae, and especially of the small mature Alophota (PI. XXVI. figs. 1-3), informs us that 

 at the very beginning the cormidia are here also more or less ordinate. A single series of a 

 few cormidia (four in Cystonula, fig. 2; six in Alopliota, fig. 3; eight to ten in Arethusa, 

 fig. 4) is here attached along the ventral median line of the trunk, and usually each of 

 these cormidia (excepting the basal group at the distal end) is composed of a siphon, a 

 palpon, and a tentacle ; and in mature corms also of a gonodendron. The internodes of 

 the trunk, or the free intervals between the succeeding cormidia, are very distinct in the 

 smaller and younger forms (figs. 2, 3), whilst they disappear in the larger and older 

 forms (figs. 4, 5). The cormidia of the former are originally monogastric (as in the 

 Rhizophysidaa), whilst they become polygastric in the latter (as in the Salacidse). 



Basal Cormidium. — The distal end of the trunk, which is the posterior in the usual 

 position of the corm (with horizontal main axis), bears in all Physalidae a separate 

 cormidium of special interest (figs. 3, 4, su). We call it the " basal cormidium," since 

 it is placed at the base or the distal pole of the main axis, opposite to the apical stigma 

 on its proximal pole (po). This primary or basal cormidium remains always sterile, and has 

 a different morphological and physiological value from the numerous secondary cormidia 

 which arise from the ventral side of the trunk and afterwards produce gonodendra. In 

 the simplest case (fig. 3) the basal cormidium consists of a single siphon (su), a palpon 

 (g), and a tentacle. The siphon placed at the very distal end, in the prolongation of 

 the horizontal main axis, has the greatest morphological interest ; it is the primary 

 siphon of the youngest larva (fig. 1), and therefore the original manubrium of the 

 primary medusome, the umbrella of which is the float ; we call it the protosiphon or 

 primary siphon, in order to distinguish it from all the other siphons, secondarily 

 produced, or the metasiphons. The primary tentacle (fig. 1, t) which belongs to the 

 protosiphon, remains either as the single tentacle of the basal cormidium, or it is after- 

 wards lost ; but I have never seen secondary tentacles developed in this distal group ; 

 usually it is composed afterwards of a series of small secondary siphons or palpons (twelve 

 to twenty or more). The interval between the basal cormidium and the larger group of 



