The Crayfish: An Introduction to the Study of Zoology.

by Thomas Henry Huxley

In the lobster, the solitary arthrobranchia of the eighth somite disappears, and the branchiæ are reduced to twenty on each side.

In Astacus, this branchia remains; but, in the English crayfish, the most anterior of the pleurobranchiæ has vanished, and mere rudiments of the two next remain. It has been mentioned that other Astaci present a rudiment of the first pleurobranchia. {266}

The branchial formula of Astacus.

So­mites and their Ap­pen­dages.	Podo­branch­iæ.		Arthro­branch­iæ.				Pleuro­branch­iæ.
			An­ter­ior.		Pos­ter­ior.
VII.	0 (ep.)		0		0		0	=	0 (ep.)
VIII.	1		1		0		0	=	2
IX.	1		1		1		0	=	3
X.	1		1		1		0	=	3
XI.	1		1		1		0 or r	=	3 or 3 + r
XII.	1		1		1		r	=	3 + r
XIII.	1		1		1		r	=	3 + r
XIV.	0		0		0		1	=	1
	6+ ep.	+	6	+	5	+	1 + 2 or 3 r	=	18 + ep. + 2 or 3 r

In Cambarus, the number of the branchiæ is reduced to seventeen by the disappearance of the last pleurobranchia; while, in Astacoides, the process of reduction is carried so far, that only twelve complete branchiæ are left, the rest being either represented by mere rudiments, or disappearing altogether.

The branchial formula of Astacoides.

So­mites and their Ap­pen­dages.	Podo­branch­iæ.		Arthro­branch­iæ.				Pleuro­branch­iæ.
			An­ter­ior.		Pos­ter­ior.
VII.	0 (ep. r.)		0		0		0	=	0 (ep. r.)
VIII.	1		r		0		0	=	1 + r
IX.	1		1		0		0	=	2
X.	1		1		r		0	=	2 + r
XI.	1		1		r		0	=	2 + r
XII.	1		1		r		0	=	2 + r
XIII.	1		1		r		0	=	2 + r
XIV.	0		0		0		1	=	1
	6 + ep. r.	+	5 + r	+	0 + 4 r	+	1	=	12 + ep. r + 5 r.

{267}

As these formulæ show, those trichobranchiate crustacea, which possess fewer than twenty-one complete branchiæ on each side, commonly present traces of the missing ones, either in the shape of epipodites, as in the case of the podobranchiæ, or of minute rudiments, in the case of the arthrobranchiæ and the pleurobranchiæ.

In the marine, prawn-like, genus Penæus (fig. [73], Chap. VI.), the gills are curiously modified trichobranchiæ. The number of functional branchiæ is, as in the lobster, twenty; but the study of their disposition shows that the total is made up in a very different way.

The branchial formula of Penæus.

So­mites and their Ap­pen­dages.	Podo­branch­iæ.		Arthro­branch­iæ.				Pleuro­branch­iæ.
			An­ter­ior.		Pos­ter­ior.
VII.	0 (ep.)		1		0		0	=	1 + ep.
VIII.	0 (ep.)		1		1		1	=	3 + ep.
IX.	0 (ep.)		1		1		1	=	3 + ep.
X.	0 (ep.)		1		1		1	=	3 + ep.
XI.	0 (ep.)		1		1		1	=	3 + ep.
XII.	0 (ep.)		1		1		1	=	3 + ep.
XIII.	0		1		1		1	=	3
XIV.	0		0		0		1	=	1
	0 + 6 ep.	+	7	+	6	+	7	=	20 + 6 ep.

This case is very interesting; for it shows that the whole of the podobranchiæ may lose their branchial character, and be reduced to epipodites, as is the case with the first in the crayfish and lobster, and indeed in most of the forms under consideration. And since all but one of the somites bear both arthrobranchiæ and pleurobranchiæ, {268} the suggestion arises that each hypothetically complete thoracic somite should possess four gills on each side, giving the following