298 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



the relations of the Tanaidce with the rest of the order are remote, and it 

 is perhaps doubtful whether they should be retained among the Isopoda, 

 especially as this family differs from the rest of the order in its mode of 

 respiration, as Avill be exijlained hereafter. 



Although this order is not a large one its representatives are perhaps 

 more widely distributed than in any other order of Crustacea. Every 

 one is familiar with ''sow-bugs" or "pill-bugs," which are found even in 

 damp houses and in cellars, as well as under leaves in woods or under 

 almost any pile of rubbish among decaying vegetable matter. These 

 terrestrial species do, indeed, become rare in the colder parts of the world, 

 but are found as far north as Greenland. Other species less familiar, but * 

 perhaps hardly less abundant, inhabit ponds and streams of fresh water, 

 and others are found along the shores of all oceans ; yet others abound 

 among the marine vegetation of the shallow waters, or fix themselves 

 upon the bodies, or within the mouths of fishes and other marine ani- 

 mals. Sp^ies are found swimming free in the open' ocean, and others 

 are brought up from the greatest depths to which the dredge has yet 

 penetrated. 



It will be convenient to give here a brief general account of the struc- 

 ture of the animals composing this order, and an explanation of the 

 terms used in their description. Most of our marine species have a 

 greater or less number of the segments at the posterior end of the body 

 coalescent, but in the genus Cirolana they are distinct; the animals 

 are, moreover, of large size and very abundant in some localities ; ref- 

 erence will therefore be constantly made to the figures of Cirolana 

 coneharum, on plates IX and X, in illustration of the parts of the animal 

 and of the terms used. A few specimens of this animal will help mate- 

 rially in gaining a knowledge of the structure of the group ; or, if speci- 

 mens of Cirolana cannot be obtained, a common "sow-bug" {Oniscus 

 or PorcelUo) may be substituted. 



The body appears to consist of fourteen segments, of which the first 

 is the head ; the next seven form the thorax, or pereion of Spence Bate, 

 and the last six the pleon, sometimes called the abdomen. Eeturning 

 to the head we find, looking from above, a pair of eyes — each consisting 

 of a group of ocelli — and two pairs of antennary organs. Of these the 

 upper i)air, or antennulte (pi. X, fig. 60), consist on each side of three 

 comparatively large basal segments, which, together, are called the 

 peduncle, or peduncular segments, and support a more slender and taper- 

 ing flagellum or lash, composed of a considerable number of short seg- 

 ments, decreasing in diameter toward the tip, and each, usually, bearing 

 a fascicle of setae, which are called by Fritz MiiUer olfactory setae, from 

 their supposed function. The antennulae are very small and rudimentary 

 in " sow-bugs" and their allies. Below the antennuhie are the antennae 

 properly so called (pi. X, fig. 61 a), which are also composed of a peduncle 

 and flagellum. The five basal segments constitute the peduncle, and the 

 following, usually much shorter and smaller segments, are flagellar. 



