Art. XXV.—Note on the Function of the Last Pair of Thoracic Legs in the Whale-feed (Grimothea gregaria).

Transactions and Proceedings of the Royal Society of New Zealand, Volume 37, 1904, Page 320

Art. XXV.—Note on the Function of the Last Pair of Thoracic Legs in the Whale-feed (Grimothea gregaria). By Chas. Chilton, M.A., D.Sc., F.L.S., Professor of Biology, Canterbury College. [Read before the Philosophical Institute of Canterbury, 30th November 1904] Plate XIX. The whale-feed, Grimothea gregaria (Fabr.), which is so common around the southern coasts of New Zealand at certain seasons of the year, belongs to a division of the Crustacea generally known as the Anomura, a group intermediate in many characters between the Brachyura (crabs) and the Macrura (crayfishes, lobsters, &c.). In the Anomura the thoracic legs of the fifth pair are generally small and weak, and thus markedly different, from the preceding pairs of legs. In giving the characters of

the Macrura anomala, a group including the majority of the Crustacea generally classed as Anomura, the Rev. T. R. R. Stebbing says,* “History of Crustacea,” p. 149. 21—Trans. “The fifth pair of legs are generally weak, not fiteither for walking, swimming, or grasping food or prey.” In many of the free-swimming forms, such as Grimothea, these legs have the joints of the limb folded against one another like the limbs of the letter Z, and the whole appendage is carried at the side of the carapace above the bases of the more anterior legs, and in this position they have the appearance of being quite useless; and I am not aware that any function has ever been assigned to the fifth pair of legs in this section of the Anomura, though in another family, the Lithodidm, where the fifth pair of legs are slender, chelate, and folded in the branchial chambers, Mr. Stebbing has suggested that they may be used to keep the branchial clear of parasites and thus be of advantage to the animal.† l.c., p. 155. During a short stay at the Marine Fish Hatchery and Biological Station at Portobello, in November, 1904, I had opportunities of observing living specimens of Grimothea gregaria, and one day while watching a small specimen under the dissecting microscope I was much interested to see the animal suddenly unfold the fifth pair of legs, stretch them forward over the anterior portion of the carapace, and with the tuft of setse on the terminal joints carefully brush away extraneous matter from the dorsal surface of the carapace, and particularly from the spaces between the rostrum, the eyes, and the bases of the antennae. The action was quite unexpected, and was almost ludicrously like that of a person engaged in brushing his back hair. For the purpose in question the fifth pair of legs of Grimothea gregaria seem well fitted—they are just long enough when unfolded to reach conveniently to the anterior portion of the carapace; and the two terminal joints (propodos and dactylus), which are bent nearly at right angles to the preceding joint, and when at rest are curved behind the bases of the fourth pair of legs, are supplied with numerous long setse projecting radially from them, one row of setae being curved and pectinate, and the whole forming a sort of circular brush specially adapted for sweeping out the spaces between the spines of the rostrum, around the, bases of the eye-stalks, &c. These two joints, moreover, form a chela or pincers with the fingers somewhat spoon-shaped, and are doubtless used to pick off substances that cannot be brushed away. As the Grimothea gregaria swims rapidly backwards by means of alternate flexions, and extensions of its abdomen,

any floating matter in the water would naturally tend to settle in these spaces, and would interfere with the proper use of the eyes; and there seems little doubt that the fifth pair of legs are of advantage to the animal in keeping these spaces clear, and in preventing parasites lodging therein. It is comparatively seldom that one has an opportunity of observing the habits of marine animals, and the use that is made in this case of a pair of appendages that have all the appearance of becoming vestigial is perhaps worth placing on record. Explanation of Plate XIX. Fig. 1. Grimotha gregaria, fifth thoracic leg (× about 15). Fig. 2. Grimothea gregaria, extremity of the same (more highly magnified).