Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.2.6. Respiratory System
Gills of lobsters are of a type known as trichobranch (TRIK-o-brank; from the Greek, thrix, hair; branchia, gills), for they are composed of numerous filaments arranged, plume- like, around a central axis. As in shrimps, on any given thoracic segment there may he as many as four pairs of gills, one pair on the basal segment of the limbs, two pairs arising from the soft membrane linking the limbs to the body, and one pair on the side of the body just above the limbs. In the American lobster the full complement of gills occurs at the base of the second, third, and fourth thoracic legs, with fewer pairs on the remaining thoracic segments except the first, which lacks gills. In all, there are 20 pairs of gills in the American lobster.

On each side the gills lie within the branchial chamber, which is formed, as in shrimps, by a deep lateral fold of the carapace. Access to the brachial chamber is through very small openings between the appendages and two larger Openings, both ventral, one at the posterior end of the branchial chamber and the other at its anterior end. In a channel at the anterior opening is the leaf-like flap known as the gill bailer, or scaphognathite, which by its rapid beating drives water forward in the channel and out of the branchial chamber.

At the same time the current thus established within the bran¬chial chamber causes water to enter the ventral and posterior openings, principally the latter. Every few minutes, the gill bailer reverses is beat for a few strokes, thereby causing the current of water to flow in the opposite direction. By this reversal of current, silt and other debris that may have settled on the gills are loosened and can be flushed from the chamber.