THE COAST OF CALIFORNIA. 19 



form, has a bell-like form with short stumpy tentacles and 

 is destitute of a i)roboscis. It has four In-oad radial tubes, 

 alternating witli other prominent structures often mistaken 

 for tubes. 



The lower side of the chitinous float is concave, in which 

 concavity lies the so-called "liver." This organ forms the 

 upper wall of the base of the polypite, and has a dark 

 brown and yellow color. Canals arise from the concavity 

 of the polypite and after anastomosing penetrate the differ- 

 ent regions of the liver, forming a "star-shaped body" in 

 the upper part of this organ. 



The liver is also penetrated by trachea^ peculiar tubes, 

 which arising from the lower plate of the float end blindly 

 in the substance of the liver. These trachete, which seem 

 to be concerned in the aeration of the fluids of the body, 

 are sometimes branched and apparently convey air from 

 the chambers of the float into the substance of a gland 

 called the "liver." By a contraction and expansion of the 

 umbrella, as described by Dr. Carl Chun, the gaseous prod- 

 ucts are expelled at intervals or introduced again through 

 these trachea}. We have in this genus an air-breathing 

 medusa, as shown by Chun, although it is proljably true 

 that there is combined with this method another found in 

 all medusae, viz. : aeration by exposure of the circulatory 

 fluids through the tissues of the body. 



The whole surface of the mantle and the membrane cov- 

 ering the "sail" in Velella are exposed to the air, and prob- 

 ably serve in the respiration of the medusa. The exposure 

 of the water-blood fluid to the air is facilitated by a nexus 

 of tubes which are found in these structures. 



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

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



1 The peculiar movements of the float of I'hysaliii in water which is impure, 

 somewliat resemble the respiratory movements recorded by Chun in Velella. 



