REPORT ON THE SIPHONOPHORyF 189 



large. Usually its form is also bilaterally symmetrical, and often divided into three 

 portions ; an odd narrow basal part, and two paired apical wings. The pedicular canal, 

 coming from the cavity of the trunk, divides at the top of the subumbrella into four 

 radial canals. Usually the two sagittal canals (shorter ventral and longer dorsal) have a 

 simply curved course in the median plane of the subumbrella, whilst the two paired 

 lateral canals right and left) are much longer and form several loops. The circular ring- 

 canal, which connects the four radial canals on the ostium of the nectosac above the 

 velum, is usually small. Many Physonectse bear a red or brown pigment-spot 

 (ocellus, ' the point where the radial canals open into the ring-canal. But usually 

 only the two lateral canals, or the dorsal also, exhibit this ocellus, whilst the ventral 

 canal has lost it (PL XVIII. fig. 9). 



Sijyhosome. — The Physonectse exhibit very great differences in the form of the 

 siphosome and in its composition from various parts. Accordingly three principal groups 

 (or suborders) may be distinguished in that great order, the Siphostelia, Macrostelia, and 

 Brachystelia. The first group, Siphostelia, are the monogastric Physonectse (Circalidse 

 and Athoridse, PI. XXI.) ; the axial trunk is represented by a single central siphon ; 

 from the superior or basal part of this (as from the manubrium of a budding Medusa) 

 arise the buds of the various medusomes, which compose the single cormidium. The two 

 other suborders are polygastric, therefore their corm is composed of numerous cormidia. 

 The second group, Macrostelia, has a long tubular trunk of the siphosome, much longer 

 than that of the nectosome, and the siphons of the cormidia are separated by long 

 internodes (Apolemidse, Agalmidse, and Forskalidse, Pis. VIII., XIV., XVIII. ). The 

 third group, Brachystelia, on the other hand, possesses a short vesicular trunk of the 

 siphosome, either a flat sac or a spirally convoluted bladder, and the cormidia are densely 

 apposed one to another, with very short internodes (Nectalidse, Discolabidse, and 

 Anthophysidse, Pis. XL, XIII. , XIX., XX.). 



The long tubular siphosome of the Macrostelia is very extensible and contractile, 

 and exhibits in the most contracted state (PL IX. fig. 6) the same appearance which the 

 Brachystelia offer permanently. The long internodes of the former (similar to those of 

 the Calyconectaa) are extremely shortened in the latter. The insertion of the cormidia, 

 however, and of the single parts composing them, is the same in both groups. All 

 parts arise originally by budding from the ventral median line of the trunk, in the same 

 way as the nectophores from the ventral line of the trunk of the nectosome. But when 

 the trunk becomes spirally twisted, then the direction of the spiral turning is usually or 

 always opposite in the two portions of the corm ; the spiral of the nectosome is mostly 

 left-handed or lambdoidal (like that of the spiral cnidobands in the tentilla) ; the spiral 

 of the siphosome, however, is usually right-handed or deltoidal. The cavity of the axial 

 trunk is continuous in both portions. The original situation of the ordinate cormidia, 

 succeeding at equal distances in the straight median ventral lino of the trunk, is 



