Branchial Formula

I have followed Burkenroad (1937b) in regarding the dorsal series of gills in Sergestes as arthrobranchs rather than pleurobranchs. Thus the branchial formula of the New Zealand specimens of S. arcticus is as follows:

	Maxillipeds			Pereiopods
	1st	2nd	3rd	1st	2nd	3rd	4th	5th
Pleurobranchiae	—	—	—	—	—	—	—	—
Arthrobranchiae	—	l	1 + l	1 + l	1 + l	2	2	—
Podobranchiae	—	1	—	—	—	—	—	—
Epipodites	1	1	—	—	—	—	—	—
Exopodites	1	—	—	—	—	—	—	—

l represents a branchial lamella (i.e., a rudimentary arthrobranch).

This formula agrees with the records, but not the terminology, for S. arcticus given by Smith (1882, 1884) and Hansen (1903, 1922) and also with Caiman's (1909: 280) formula for the genus Sergestes.

In the New Zealand specimens the anterior arthrobranch of the 3rd pereiopod is nearly twice the length of the anterior branch of the 4th. The posterior arthrobranch of the 3rd pereiopod, clearly not rudimentary, is larger than the posterior branch, and only a little shorter than the anterior branch, of the 4th pereiopod.