Then he describes the method in which the Amphipod works up bits of weed and its own droppings into the framework of the tube. In putting its foecal pellets to this use, it reminds one of a species of Melicerta (Melicerta janus)[52], which employs the same material to coat its gelatinous sheath.
In breaking up weed and pellets with its foot-jaws and (probably) its mandibles, the Amphipod recalls the practice of some of the Masking Crabs, which have been seen to apply to the mouth the material they were using to deck themselves. Dr. Aurivillius suggests that in the case of the Crabs there may be an adhesive secretion from the mouth, as there is possibly in the Amphipods. ‘The spinning was done wholly with the first and second pereiopods, the tips of which were touched, from point to point over the inside of the skeleton tube, in a way that recalled strongly the movements of the hands in playing upon a piano. The cement adhered at once to the points touched, and spun out between them in uniform delicate threads. The threads seemed to harden very quickly after they were spun, and did not seem, even from the first, to adhere to the animal itself. In one case, in which the entire construction of the tube was watched, the work was apparently very nearly or quite completed in little more than half an hour.’
The species we are likely to meet with in rock-pools fashion their tubes in a similar way. The only difference to be noted is that they employ less cement, and a larger proportion of broken-down weed and other matters.
The Sand-hopper (Tali´trus locus´ta) and the Shore-hopper (Orches´tia littorea) are so exceedingly plentiful that it may be well to collect and preserve some during any visit to the seaside. Both are of fairly large size, and present no great difficulty to us in making out their several parts. Let us take the Sand-hopper first.
Sand-hoppers swarm on most sandy shores, where they perform the useful part of scavengers. They are always found above high-water mark, and do not enter the sea of their own accord. In hunting for them it is a good plan to turn over decaying masses of sea-weed, for under them the Sand-hoppers are sure to swarm.
Strange tales have been told of their voracity. Bate and Westwood[53] record the story of a correspondent who says that at Whitsand he ‘saw “not millions, but cartloads,” of this species lying piled together along the margin of the sea. They hopped and leaped about, devouring each other, as if for very wantonness. A handkerchief, which a lady let fall amongst them, was soon reduced to a piece of open work by the minute jaws of these small creatures.’
This statement has been copied into a good many books, without criticism. At last Mr. David Robertson tried various experiments with a view to discover if these creatures would feed on each other, or, failing other food, put up with cambric or muslin. The specimens upon which he made his observations did neither the one nor the other. Mr. Robertson embodied the results of his experiments in a paper which he read before the Natural History Society of Glasgow[54]. And the story may be read in an abbreviated form in the Rev. T. R. R. Stebbing’s Naturalist of Cumbrae, p. 329.
In Gammarus we have a standard with which to compare our Sand-hopper. The first thing to notice is the difference in the antennae. Here the superior pair are very short, and carry no secondary appendage, and the lower pair have no denticle or tooth-like process. There is also considerable difference in the gnathopods, or jaw-feet, the sixth joint of which, in the Sand-hopper, does not form a ‘hand.’ The pleopods, or swimming feet, are small, and are used for leaping. We shall also find some difference in the details of the mouth parts, especially in the toothing of the mandibles.
We now come naturally to the Isop´oda, which are distinguished by the nearly uniform size of the seven segments which constitute the trunk, and the seven pairs of limbs borne by these segments. The head is distinct, and the breathing apparatus is carried on the under side of the pleon—in these animals not the ‘swimming’ part—five pairs of plates lying one over another, sometimes covered by a larger outer pair.
A normal I´sopod may be represented