92 THE FAUNA OF THE DEEP SEA 



Like most of the deep-sea Hydroids, the Pectyl- 

 lidas are usually devoid of sense organs, but a single 

 specimen of Periphylla mirabilis, captured by the 

 naturalists of the ' Challenger,' possessed well-marked 

 eyes. 



There is also a peculiar family of the Siphonophora, 

 called the Auronectas, consisting of a few specimens 

 that have been hitherto found only in very deep 

 water. Like the well-known Portuguese man-of- 

 war Physalia of the surface waters, the Auronectae 

 possess a large swimming bladder or pneumatophore, 

 but they have in -addition another peculiar bladder-like 

 cavity, called the aurophore, communicating with it, 

 which may be an organ for secreting gas. 



A very interesting genus allied to Velella was also 

 found in depths of over 2,000 fathoms by the ' Chal- 

 lenger ' expedition. It is supposed to be a survival 

 of the ancestral form of the Disconectas, or, at any 

 rate, to be a link connecting the Siphonophora with 

 the Medusas. The very well marked octoradial 

 arrangement of the parts of Discalia, as this genus 

 has been termed, is certainly a point of great interest 

 and importance. 



There is no large family of the sea anemones that 

 is peculiar to deep water, but several genera that 



