IV 



COELENTEKATA 



65 



or who has seen some members of a swarm of Ascidian tadpoles thus 

 fix themselves to the film. The surface film, although able to sustain 

 the weight of a larva, would soon bend under the growing weight of 

 the hydroid colony which developed from it, and this would lead to 

 a cupping of the base. If we suppose this base to secrete mucus and 

 to entangle bubbles of air, the elements of a float would thus be 

 presented. 



FIG. 43. Three stages in the development of a Siplionophore (CystaUn inn,i,,iinstrica). 



(After Haeckel.) 



A, planula with Boat, an open inva^iuation of the aboral ectoderm. B, Older larva, a single 

 tentacle formed. C, still older larva in whirh the definitive endoderm is formed, ami in \vhk-h 

 buds of other persons have been formed, b, buds ; end, endoderm ; jjn., float ; ten, tentacle. 



The earlier stages in the development of these Siphonophora 

 have not been made out, but the planula larva is well known. The 

 peculiarity of this larva lies in the fact that the large vacuolabed 

 internal cells which occupy its interior, are not directly converted 

 into the endoderm of the adult, but that they bud off smaller cells 

 on their outer sides, which form the definitive endoderm which 

 persists throughout life. 



As in other planulae, the narrow end lengthens and becomes 



VOL. I F 



