REPORT ON THE SIPHONOPHOR^. 345 



ventral cormidia is in most Pl^salidse permanent, sometimes larger, at other times 

 smaller, than in Alophota giltschiana, fig. 3, Physalia pelagica (77, Tab. xxxv. fig. 2), &c; 

 it is lost afterwards in those species, in which the number of cormidia is exceedingl)- 

 augmented, and all are confluent in a single crowded mass (as in Aretkusa challenger!, 

 figs. 4, 5, in Caravella maxima, and in Physalia megalista). 



Ventral Cormidia. — The numerous secondary cormidia which compose the large 

 main group of appendages, on the ventral side of the trunk, exhibit a great variety in 

 number, size, composition, and arrangement. Possibly these differences are constant in 

 different localities and possess therefore a systematic value ; but they recmire a far more 

 accurate anatomical examination than has been employed hitherto. Originally each 

 secondary cormidium (in most species at least) seems to be monogastric, composed of a 

 single siphon, a palpon, and a tentacle, and in small mature corms of a gonodendron 

 (fig. 3) ; but usually the common pedicle of these medusomes afterwards branches, and 

 produces a variable number of tertiary cormidia. Generally a single tentacle and the 

 appertaining palpon, in the middle of the ventral group, becomes early much larger than 

 all the others ; this predominant main tentacle remains single in Alophota and Physalia, 

 whilst a variable number of similar gigantic main tentacles (usually ten to twenty) is 

 afterwards produced in Aretkusa and Caravella. Many secondary cormidia remain sterile 

 in most Physalidee, and a small number only (usually eight to twelve) develop a large 

 gonodendron. It may be, perhaps, that in the crest-bearing Physalidas (Physalia and 

 Caravella) the number of large primary air-chambers in the crest (usually eight to twelve) 

 and their metameric succession often correspond to the segments of the trunk, from 

 which arise the primary groups of ventral cormidia. 



Siphons. — The feeding polypites exhibit in all Physalidse the same shape, and are 

 very similar to those of the Epibulidge (PI. XXII. fig. 6) and Salacidse (PI. XXV. fig. 5). 

 The protosiphon (or the primary polypite of the basal cormidium) does not differ in 

 structure from the numerous metasiphons (in the secondary cormidia of the large ventral 

 group). The young siphons are simple spindle-shaped tubes, whilst the fully developed 

 exhibit distinctly three or four different segments (PI. XXVI. fig. 6). The two proximal 

 segments, viz., the thin pedicle (sjy) and the vesicular basigaster (sb), are usually small, 

 and often confluent ; the two distal segments, however, are always large and distinct. 

 The stomach is a very dilatable sac, inside covered with numerous black hepatic villi 

 (sv); the proboscis is a very muscular cylindrical tube, very contractile and expansible, 

 and opens by a mouth, which may be expanded in the form of a circular or polygonal 

 (often square) suctorial disc (fig. 1, ss) ; its margin is armed with a series of cnidocysts 

 (compare on the structure of the siphons, Leuckart, 81, Huxley, 9, &c). 



Palpons. — All Pkysalidse possess, intermingled with the mouth-bearing siphons, a 

 larger number of mouthless palpons. These are of two kinds. The first kind exhibits 

 the same structure as the siphons and differs only in the absence of a distal mouth-opening. 



(ZOOL. CHALL. EXP PART LXXVII. — 1888.) Hhhh 44 



