CRUSTACEANS ness of a smooth black shining pill. These usually belong to the second of the families above mentioned, and the habit in question would almost suffice alone to affiliate them to it. But a certain caution must be observed, first because in the other family the members of one genus are nearly as perfect in the globe-making practice, and secondly because among myriapods there is a genus quite as perfect in this defensive manoeuvre and so similar also in colour and smoothness that the rolled up specimens are often mistaken for the true woodlice, which are not myriapods but crustacean isopods. Another distinction consists in the circumstance that the young of the Oniscida? quit the mother while the seventh segment of the middle body is still undeveloped, whereas the young of the other family have that segment already developed when they first enter upon an independent career. For perceiving this differ- ence however a careful examination is required. The young ones, it is true, are often extremely abundant, especially among the Oniscidae, but they are pallid and small. A distinction more easy to observe will be found in the tail part of the adults. This part, otherwise known as the abdomen or pleon, appears to consist of six segments instead of the theoretical number seven. The terminal segment or telson is coalesced with the preceding segment which carries the last pair of appendages, known technically as uropods or tail-feet. In the Oniscidas it will be without difficulty discerned that the outer branch of these uropods is prolonged beyond the compound telsonic segment. In the other family the tail-feet and tail-segment are so arranged as to form a neat unbroken curve which without any projecting parts can meet the front of the animal for spherical adjustment. There are some ungifted persons who may think that all this is ' much ado about nothing,' and certainly whoever first devised the names Oniscus ase//us, adopted by Linnaeus for the captain of all the woodlice, must have attributed to that species the anxiety so emphati- cally expressed by Shakespere's Dogberry, ' Masters, do not forget to specify, when time and place shall serve, that I am an ass.' Not only does the Linnasan designation first in Greek and then in Latin libel the species as a little donkey, but also our own celebrated countryman Ray had before the time of Linnaeus called it Asellus asininus, as though willing to translate into scientific language the impatient excla- mation of Conrade in the play, ' You are an ass, you are an ass.' For a considerable time the genus Oniscus was equivalent to the Isopoda at large, which are now distributed among several tribes, many families, and very numerous genera. To Oniscus remains only a shadow of its past supre- macy in the form of the family name Oniscida? and the tribal name Oniscidea. To the genus itself very few species are now assigned. The eldest and commonest, Oniscus asellus, Linn., is probably known to every one by sight though not by name, being abundant in gardens, in woods, and in almost all places where it can find convenient shelter with food and moisture. From its frequency on the walls of cellars and other damp places it has been by way of alternative called O. murarius. The other i 105 14
