REPORT ON THE SIPHONOPHOR^E. 23 



" bilateral," sometimes even " uniradial " (Chun). But a comparative study of their 

 development, and mainly of the central part of their body, demonstrates that the corm 

 of all Velellidae originally is also octoradial ; this is clearly proved by the girdle of eight 

 radial lobes which surrounds the central chamber of the pneumatocyst ; further, by the 

 eight primary radial canals, which arise from the gastrobasis of the central siphon and 

 give off the ascending branches forming an octoradial liver-star on the upper surface 

 of the centradenia. Rataria (PI. XLIV.), as the simplest form of this family, and the 

 similar Ratarula-larvse of other Velellidae, possess marks of an octoradial type also in 

 other organs (eight primary buds of gonostyles, sixteen primary tentacles, &c). But 

 whilst the eight parameres in the Discalidae and Porpitidse are precisely equal (each 

 composed of two symmetrical antimeres), they are unequal in the Velellidae (each 

 composed of two asymmetrical antimeres). The two horizontal cross-axes, which are 

 perpendicular one to another, and to the vertical main axis, are both equal in the 

 Discalidae and Porpitidse ; whilst they are unequal in the Velellidae. Their sagittal axis 

 (marked by the vertical sail) is longer than the frontal axis (marked by the transverse 

 groove of the pneumatocyst). But the dorsal half of the corm (on one side of this 

 frontal groove) is symmetrically equal to the ventral half (on the opposite side), in the 

 same manner as the right half is symmetrically equal to the left (separated from it by 

 the vertical sail). The fundamental form of the corm in the Velellidae, therefore, is not 

 "bilaterally-symmetrical," but " amphithect," or bilaterally-octoradial, similar to that of 

 the Ctenophorae. 



The octoradial structure of the Medusae is originally derived from the quadriradial 

 type, as I have demonstrated in my Monograph of the Medusae. The same promorpho- 

 logical law is valid also for the Disconectae. Their octoradial trunk has arisen from the 

 umbrella of some Medusa, the older ancestors of which were quadriradial. Their next 

 ancestral forms may be Trachynemidae, with eight radial canals, eight tentacles, &c. 

 But these again have arisen from the older Petasidae, which possess four radial canals, 

 four tentacles, &c. This older quadriradial structure is still preserved in the medusiform 

 gonophores of the Disconectae (Discomitra), 



Promorph of the Corms in the Siphonanthse. — The corm of the Siphonanthae, 

 differing from that of the Disconanthae in all respects from the first beginning, is also 

 distinguished completely by its bilateral promorph. The primary larva (Siphonula), 

 which develops the corm of the Siphonanthae by unilateral budding from its manubrium, 

 has already a markedly bilateral fundamental form. Its primary umbrella has a deep 

 ventral cleft, and its only tentacle is attached to one side of the manubrium. The 

 vertical plane, which passes through the median line of these parts, is the sagittal plane, 

 and bisects the entire body ; the two halves separated by it, right and left, are 

 symmetrically equal. That side of the manubrium from which arise the buds of the 

 corm is the ventral side, the opposite the dorsal side. 



