116 NEW INVERTEBRATA FROM 



In the Portuguese Man-of-War 1 in which the float has the 

 form of a huge bag, the feeding-polyps being clustered on 

 the submerged portion, we probably have a similar respi- 

 ration by direct contact with the air through the walls of 

 the float. In the genus Physalia there is an opening into 

 the float by which air can enter its interior so that there 

 may be a double exposure, inside and out. Among the 

 Rhizophysidse we have appended to the under surface of 

 the enclosed air-sac a number of finger-like appendages, 

 often branched, which convey the air into the cavity of 

 the stem of the animal, so that their walls alone separate 

 the air from the fluid. These structures are possibly or- 

 gans of respiration comparable with the tracheae of Velella. 



Among those Physophores, however, which have nec- 

 tocalyces and covering-scales the function of respiration is 

 probably accomplished, as in all medusae, by exposure of 

 the outer surface of the body to the water. In Siphono- 

 phores, where the nectocalyces are absent, the float is en- 

 larged or the covering scales are well developed. 2 



In the family of Forskalidss, which move very rapidly 

 and in which respiration must on that account be some- 

 what active, the spread of covering-scales and nectocalyces 

 is very large, but the float is very small. In Calycopho- 

 ridse, the motion of which is the most rapid of all these 

 animals, covering-scales are often very prominent. A dim- 

 inution in the size of both nectocalyces and covering- 

 scales is accompanied by an enlargement of the float and 

 a more sluggish habit of life. 



1 The peculiar movements of the float of Physalia in water which is impure, 

 somewhat resemble the respiratory movements recorded by Chun in Velella. 



*No satisfactory explanation of the physiological role of the covering-scales has 

 yet been suggested. I believe that they are respiratory bodies which may some- 

 times perform also as in Athorybia the function of locomotion. Wherever they 

 are wanting their respiratory function is performed by the swimming bell (the 

 float is a modified nectocalyx), when the medusa swims below the surface or by 

 an enlarged float when aerial respiration occurs. 



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