The outer ring of arms and tentacles is the brachial crown. The inner ring formed by the buccal membrane and buccal supports is the buccal crown. The lateral keel of arm IV is also known as the "tentacular sheath."
In this illustration, the buccal connectives attach to the dorsal margins of Arms I and II, and to the ventral margins of Arms III and IV (i.e., the formula is DDVV). Buccal suckers are found in only a few decapodiform families. The number of buccal supports varies from six to eight.
The primitive number of buccal supports was probably eight. In most decapodiforms this number has been reduced to seven by the fusion of the two dorsal supports (supports I). In a few groups, this number is further reduced to six, as the two ventral supports (supports IV) have become fused.