METAZOA COELENTERA. 109 



SlPHONOPHORA. 



There is a good reason for placing the Siphonophora as 

 the most specialized group of the Hydrozoa, since the proc- 

 ess of budding which we have found in the larval Hydra 

 is carried back to a still earlier stage and exists in the 

 embryo itself. This precocious germ develops into a com- 

 plex colony of hydra-like and medusa-like zoons. 



Velella mutica Bosc (No. 142) in addition to a float has 

 a triangular sail. According to A. Agassiz, Velella has 

 much in common with the Tubularians, the young medusa 

 resembling in a marked degree the medusae of that group. 



In the adult Velella a single feeding zoon extends 

 downward from the lower side and around it are many 

 small appendages in the form of delicate threads which 

 bear tiny medusae buds ; these separate and swim away. 

 The float above is surmounted by the sail (finely seen in 

 the alcoholic specimen, No. 142) which, according to 

 Agassiz, 1 is left handed ; that is, the sail runs northwest 

 and southeast, the longitudinal axis of the float being 

 placed north and south. In 2500 specimens thrown on 

 the beaches at the Tortugas the position of the sail was 

 the same, showing that this character has become so 

 firmly fixed in the organization that it is not subject to 

 variation. 



Porpita I'mnaeana Less. (No. 143; No. 144, preserved 

 specimen; No. 145, model of P. umbrella Esch.), proba- 

 bly possesses a sail in youth which is lost in maturity. 

 It has a central disc, the upper side of which is corru- 

 gated. ' The internal structure is somewhat complex. 

 Long and short tentacles extend from the edge of the disc 

 (No. 145), and these are provided with knobs which can 

 be seen flattened in the preserved specimen (No. 144). 

 Near the disc and at the base of the tentacles are the 



1 Mem. Mus. Comp. Zool., VIII, 1883. 



