THE BURROWING BARNACLES (CIRRIPEDIAI ACROTHORACICA ) 17 



The beating rates seem to be the result of a feeding action, which 

 can beat at any given rate, or cease completely. It is not a matter of 

 irritation of the animal. The barnacles have been seen stroking after 

 being completely untouched for hours, even days; yet they also have 

 been completely quiescent after equivalent periods of time. 



Digestive System and Feeding Mechanisms 



All acrothoracicans collect food in a manner similar to the thora- 

 cicans, that is, with a sweeping of a cirral net. Generally the cirral 

 net extends out through the mantle aperture and beyond the burrow, 

 sweeping in a rhythmical manner, in much the same way as thoracican 

 barnacles sweep the water with their cirri. These species are aligned 

 vertically to the surface of the shell, and are equipped with long 

 biramous cirri — much like those of thoracican barnacles. I have ob- 

 served the following species behave in this manner : Weltneria spinosa, 

 W. hirsuta, Lithoglyptes spinatus, Kochlorine floridana, and Crypto- 

 phialus melampygos. 



An interesting comparison of the feeding mechanism of these species 

 with the species of the genus Trypetesa can be made. Trypetesa lateralis 

 feeds with regular sweeps inside the mantle cavity and the water 

 currents set up by the sweeping bring food material into the sac and 

 within the area covered by their rather reduced terminal cirri (see 

 Tomlinson, 1955). The shortness of the thoracic cirri makes the for- 

 mation of a fan-like net of cirri impossible in the genus Trypetesa. 



This leads us to the speculation that the shortening of the thoracic 

 cirri is perhaps a modification due to the occasional, somewhat hori- 

 zontal orientation of most trypetesids in the thin regions of the shell, 

 and to the completely horizontal orientation of T. lateralis. A hori- 

 zontal canting, even though sUght, would greatly impair the activity 

 of the rapidly sweeping fan seen in the vertically aligned forms. 



Many specimens of some trypetesids {T. lampas, T. habei) are 

 oriented in a completely vertical fashion, so the modification of the 

 cirri would be adaptive only to those specimens occupying thinner 

 regions of the shell. Once the feeding mechanism was perfected in 

 primitive trypetesids, the more horizontal orientation of the specialized 

 T. lateralis required only the adaptation of a separate water circulation 

 for the males, via the external mantle flap. 



Since the terminal cirri are clustered at the posterior end of the 

 thorax (hence the name "acrothoracica") they do not move in relation 



