30 NATURAL SCIENCE. January. 



their canal system, viz : — the ascons, the sycons, and the leucons, 

 forming three stages in an ascending scale of differentiation. Now 

 Sollas and other competent spongiologists consider that the leucon 

 type has been evolved many times amongst the different families 

 into which the sycons are divided. The horny sponges and the 

 sponges devoid of spicules have likewise had in each case two or three 

 roots. Among the echinoderms we meet with similar cases on every 

 hand. One of Perrier's main divisions of the asterids is that of the 

 Valvata, in which the plates which form the skeleton are covered 

 with a uniform granular coating, there being practically no spines. 

 This division includes long-armed and short-armed forms ; the latter 

 glide by insensible gradations into the short-armed forms of the group 

 Paxillosa, in which the armature is composed of circles of spinelets 

 borne on a button, of which Astvopecten is the best known example. 

 The long-armed forms probably have no affinity with Paxillosa. The 

 term " short-armed " is not quite correct ; it is not only that the radii 

 are comparatively short, but that the arms have coalesced with each 

 other laterally. Amongst the ophiurids, the habit of carrying the 

 young in the genital bursse, which gives rise to great differences in the 

 development, has been independently acquired in four or five distinct 

 genera, other species of which give rise to free-swimming larvae. 

 Among echinids the eccentric position of the anus, and the peculiar 

 modification of the ambulacra, involved in their assuming the petaloid 

 form, when the tube-feet become broad respiratory leaves, is found in 

 the Clypeastridas and Spatangidae. As the first group retain the jaws 

 of the regular echinids whilst the second have lost them, one might at 

 first sight infer that the spatangids were merely a further development 

 of the clypeastroid type, but this supposition is precluded by the fact 

 that in both groups we find fossil forms with perfectly normal 

 ambulacra and almost central anus. Among the annelids, the group 

 of the tubicolous annelids is now divided up into families which 

 are placed with families of the Errantia. Amongst arthropods, those 

 air-breathing arachnids which retain lung-books, the Pedipalpi on the 

 one hand and the scorpions and spiders on the other, have, according 

 to Laurie, been derived from the water-breathing eurypterids along 

 two distinct lines. The land-crabs, which have so completely acquired 

 the air-breathing habit that they die when immersed in water, have 

 been derived from various marine ancestors. The molluscs offer 

 perhaps the best examples of all of homoplasy. The pteropods, once 

 supposed to be a most clearly defined order, are now admitted to have 

 been derived along two lines from the opisthobranchs ; the Thecoso- 

 mata from Bulla-V\ke forms, and the Gymnosomata from nudibranch 

 genera. In the newest classification of the Pulmonata, the two 

 common slugs Avion and Limax are separated from each other, the 

 first is classed in the same family as Helix, and the second is made 



1 M. Laurie, " Anatomy and relation of the Eurypterida." Trans. R. Soc. Edin- 

 burgh, vol. xxxvii., pp. 509-528, 1S93. 



