XXX.—Experiments and Observations on Crustacea. Part VII: Some Structural and Physiological Features of the Valviferous Isopod Chiridotea

Abstract

Summary1. Following upon a previous study of spirit specimens of Glyptonotus, observations have been made on live examples of Chiridotea cœca and C. tuftsii with the object of elucidating points relating to the natural history and structural peculiarities of the Glyptonoteinæ.2. Though wholly carnivorous, Chiridotea is not a predaceous animal. Subsisting on portions of animals, already dead, which it discovers by persevering search aided by sense of smell, it is a scavenger rather than a hunter of living prey.3. The uncommon division of the thoracic limbs in Chiridotea is associated with an exceptional range of locomotor accomplishments. The animal not only walks and swims like other members of the Idoteidæ, but also tunnels its way for long distances under the surface of sand.4. The three pairs of gnathopods subserve a number of functions. They are used in ordinary reptant progression. They are used to seize and hold on to food, but not to tear or rend it. Working in conjunction with the mandibles, they play an essential part in the process of manducation. They also come into action during sand-tunnelling.5. On closer observation the characteristic type of limb-taxis of Chiridotea proves to be specially related to the tunnelling habit. During the process of tunnelling the mesially situated gnathopods and pleopods, working together, deal with one body of sand, the laterally projecting peræopods with another.6. Certain common features in the bodily conformation of Serolis, of Apus, of Limulus, and of Trilobites are interpreted as an adaptation, by convergence, to a habitat on the surface of mud.7. Owing to the transparency of its body, the movements of the alimentary canal of Chiridotea can be observed during ingestion and the movements of the hepatic cæca during digestion of food.8. The midgut exhibits, according to circumstances, two different types of peristaltic movement. At the very commencement of a meal, and also during long continued vain effort to masticate a tough morsel of food, a succession of forward-running waves of contraction is observed. When food begins to be swallowed these are replaced by backward-coursing waves of contraction.9. There are two pairs of hepatic cæca. As the midgut fills the more mesial pair passes out of sight. The laterally situated cæca increase greatly in volume and exhibit a series of dilatations or pockets, which enter into vigorous and long-sustained rhythmic contraction.10. The mesial split in the thoracic sternites is a device for allowing distension of the body after a meal.11. Its eyes being wholly dorsal, Chiridotea shows no conspicuous colour change in response to its background. It has both brown and black chromatophores, the latter contractile.12. A new point of specific difference between C. cœca and C. tuftsii is recorded.13. The direction of rotation by which the uropods of Valvifera have assumed their present orientation is discussed anew, the conclusion again being that the anterior border was originally mesial.14. Objections are raised against the elevation by Racovitza and Sevastos of the northern representatives of Miers' Glyptonoteinæ to independent sub-family rank.