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PHYLUM CCELENTERATA 



165 



obviously corresponding with the float of Physalia : each of the 

 chambers communicates with the exterior by a couple of pores. 

 The edge of the disc is beset with long dactylozooids (t), and from its 

 lower surface depend numerous closely set blastostyles provided 

 with mouths and bearing medusae, while in the centre is a single 

 large gastrozooid (hy). The mouth of the gastrozooid leads into 

 a wide gastric cavity. Between this and the pneumatophore is a 

 thick cellular mass through which ramify numerous canals belonging 

 to two systems. One of these systems is endodermal : it com- 

 municates with the endodermal cavities throughout the colony. 



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Fio. 125. Porpita pacifica. A, from beneath ; B, vertical section, hy, large central 

 gastrozooid : hy', blastostyles ; sh, chambered shell ; t, dactylozooids. (From Parker's 

 Biology, after Duperry and Koelliker.) 



The other (tracked system) is ectodermal and is lined internally 

 throughout by a chitinous layer ; it opens by numerous apertures 

 into the air-chambers of the float. The closely allied genus Velella 

 is of rhomboidal form, and bears on its upper surface an oblique sail. 

 The reproductive zooids are liberated as free medusae. The 

 eggs give rise to young which have a close resemblance to flat 

 medusae with manubrium, marginal tentacles, and an air-chamber 

 or float developed in the ex-umbrella. Thus it is quite possible 

 that the Siphonophora of the Porpita-type may be medusae the 

 sub -umbrella of which has given rise to buds forming the feelers 

 and blastostyles. But, as their early development is not known, it 



