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



permanent in two remarkable genera of the Agalmidse, Crystallodes (PI. XVII.) and 

 Stephanomia (Amphitrites, Peron). The horizontally swimming trunk is here so densely 

 covered with thick prismatic bracts, that it becomes rigid and almost every contraction 

 is excluded, no spiral turning possible. 



Cormidia. — The arrangement of the polymorphous medusomes in the Physonectse, 

 their composition and metameric succession along the stem, offer numerous important 

 modifications, hitherto much neglected. The general opinion has been up to this time, 

 that in most Physonectse the polymorphous persons and their organs are intermingled 

 without definite order, and that only in a few cases, as in Apolemia and Physophora, 

 they may be disposed according to certain laws, as in the Calyconectse (8, p. 311, &c.). 

 But in reality the reverse is the case. The. great majority of Physonectse possess ordinate 

 cormidia, and in the minority only irregular cormidia with more or less scattered parts 

 occur. The cormidia are ordinate and separated by free internodes of equal length in the 

 Apolemidas, and Stephanomidse (the first subfamily of Agalmidae) ; they are also ordinate, 

 but separated by narrow internodes or constrictions of the trunk, in the Nectalidaj, 

 Discolabidse, and Anthophysidse. On the other, hand the cormidia are irregularly 

 placed, and the polymorphous parts scattered along the stem, in the Forskalidse (except 

 Strobalia) and the Halistemmidee (the second subfamily of Agalmidas). 



The cormidia of nearly all Physonectse are monogastric, with a single siphon and a 

 single tentacle attached to each node of the trunk. A single exception is formed by 

 Apolemia and the closely allied Apolemopsis ; these possess two to four or more siphons 

 in each ordinate cormidium, and the same number of tentacles. 



Bracts. — Nearly all Physonectse possess a great number of hydrophyllia, bracts or 

 covering scales ; they are wanting in two families only, the Circalidse and the Discola- 

 bidse ; in the latter they are probably lost by phylogenetic reduction. Usually the 

 bracts cover the entire stem in hundreds or thousands, and arise not only from the 

 nodes of the siphosome, but also from the internodes between them ; or even from the 

 pedicles of the siphons (Forskalidse) ; but sometimes they arise only from the nodes. 

 Their general function is that of protective organs, or shields, which cover the other parts 

 of the corm ; in two families, however, the Athoridse and Anthophysidae, where necto- 

 phores are wanting, the bracts take on besides a locomotor function and become paddling 

 organs (PI. XXI. fig. 5 ; PI. XL fig. l). Sometimes the distal end of the bract in this 

 case encloses a small rudimentary suhumbrclla, and is thus proved to be developed from 

 a reduced nectophore (PI. XXI. fig. 6). At other times it is possible that each bract 

 may not be homologous with a complete umbrella, but a separate portion of a cleft (e.g., 

 quadripartite) umbrella. Their jelly-substance is always colourless, hyaline, and trans- 

 parent, usually rather firm, often cartilaginous. Their form is extremely variable. The 

 bracts in the majority of Physonectse are rather thin scales, lanceolate or triangular 

 plates, more or less foliaccous or squamiform, with a convex upper (or dorsal) and a 



