THE BURROWING BARNACLES (CIRRIPEDIA: ACROTHORACICA ) 19 



The Cirri 



The number of cirri is the most important single characteristic in 

 the order Acrothoracica. The new genera and families have not always 

 been established primarily with the number of cirri in mind, but it 

 happens that this is the single most discrete differentiating charac- 

 teristic of such groupings. Although it may prove that the separation 

 of genera by the number of cirri is not well founded, I have retained 

 it as the most successful method. I would caution the reader to make 

 certain of the number of cirri very carefully, as it is easy to miss a 

 pair when they are only about a millimeter long, usually less than 40 

 micra wide, and are bunched together in a tight mass. I have erred 

 twice in this myself, and I urge workers in this group to make cirral 

 segment counts, not so much for the segment count itself, but to 

 better insure an accurate cirral count. 



Referring again to the segment count, it seems that knowledge of 

 the number of segments is of little value in the identification of the 

 Acrothoracica. Not only are the numbers difficult to ascertain, but are 

 highly variable between the two sides of one animal, and between 

 different animals in the same host. A count, nevertheless, will be given 

 for one side of one animal where possible. 



The setation, or the size and distribution of bristles on the cirri, 

 is very characteristic. The setae themselves may be faintly haired, or 

 "plumose." This characteristic is less useful than the size and distri- 

 bution of the setae on the cirrus, inasmuch as we are now at the 

 limits of resolution of a light microscope. 



Feeding Habits of Trypetesa lateralis Females 



The females of the genus Trypetesa have no anus. This deficiency 

 is amplified by the complete absence of an intestine, as well. The 

 digestive tract of these barnacles is composed of a mouth, a short, 

 highly muscular esophagus, and a large sack-like stomach with weak 

 musculature. 



In studying the feeding habits of the female T. lateralis, the 

 question arose that if she took indigestible items into her stomach, how 

 did she get rid of them? There are several possibilities: First, she 

 need not take indigestible items into her stomach. As a second hypoth- 

 esis, if she does ingest these items, perhaps she does not get rid of 

 them, but accumulates them until she dies. Thirdly, she may actually 

 void the contents through the body wall by some method of phago- 

 cytosis. Or, fourthly, she could regurgitate these items back through 



