SCOTTISH TARDIGRADA, COLLECTED BY THE LAKE SURVEY. 649 



Segment VI is regarded as all that remains of the abdominal segments of other 

 Arthropods. Originally Professor Richters considered as segment VI the middle lobe 

 of what I have called the lumbar plate, but the discovery of many species having three 

 pairs of plates has led him to modify his views. He now regards this type of struc- 

 ture as more primitive than the commoner type. E. islandicus (fig. 4a) is a good 

 example of it. Segments III, IV, and V each bear pairs of dorsal plates ; segment VI 

 is three-lobed. In the commoner type of structure (Plate I. fig. 5) only III and IV 

 bear paired plates. After IV comes a single large three-lobed plate, like segment VI 

 of E. islandicus, which is regarded as being composed of V and VI, so fused together 

 that their limits are indistinguishable. The species conforming to the more primitive 

 type, with V and VI distinct, are E. arctomys, E. mutabilis, E. conifer, E. islandicus, 

 and a number of other species still undescribed. 



All species are easily referable to one or other of these types of structure. Minor 

 differences consist in the subdivision of certain plates, or the intercalation of smaller 

 plates among the larger ones. The first and second median plates are often divided 

 transversely into two more or less distinct plates. Plates II and VI are in some species 

 divided by both longitudinal and transverse plain bands, but the "panels" thus formed 

 can hardly be regarded as separate plates. 



Texture of Plates. — Nearly all the known species have some kind of surface 

 markings. In many species these are granules, and it has been usual to refer to any 

 markings as granules. There is no doubt that in many cases they are not granules, 

 though their true nature is difficult to make out. If Echiniscus spitsbergensis is 

 turned about into various positions, so that the markings may be seen in profile, it 

 becomes evident that they do not project. 



In many species the markings look like perforations, and in descriptions I refer to 

 them as such, using the terms cribrose or pitted ; but in these cases I am dealing with 

 appearances, and am not yet satisfied as to the nature of the dots. They may be of 

 different texture from the rest of the skin, and only become perforations when the skin 

 is cast, yet many have this appearance during life. 



A few species are reticulate on the dorsal plates. 



In E. reticularis the pattern is very regular, the lines separating the hexagons 

 being merely the very slightly raised margins, the spaces themselves being depressed. 

 The reticulation of E. islandicus is of another nature. The spaces are of unequal sizes, 

 generally irregular polygons, and they are bounded by pearly dots, of unknown nature, 

 sometimes in double or treble rows between the spaces. In both species the reticulation 

 dies out towards the margins of the plates. 



Processes. — In framing diagnoses of species, much reliance has been placed on the 

 various dorsal and lateral spines and setae. As superficial processes are often the most 

 variable of characters, it may be thought that too much weight had been given to such 

 peculiarities ; and it is true that, if the processes vary in number, there is little or no 

 other distinction between most of the earlier-described species. The distinction of 



TRANS. ROY. SOC. EDIN., VOL. XLV. PART III. (NO. 24). 92 



