The petioles have a somewhat complex structure, the bundle often having, in transverse section, the form of an H; it has been proposed to subdivide the genus on the details of the petiolar structure.
The diversity of scales appears to be due mainly to four causes: (i) the tendency to group into scores (§ 20); (ii) the tendency to subdivide into twelve; (iii) the tendency to subdivide into two or four, with repetitions, making subdivision into sixteen or sixty-four; and (iv) the independent adoption of different units for measuring the same kind of magnitude.
It is perhaps safer to subdivide the Order into 6 Suborders (in the number of these following Benham, except in combining the Sabelliformia and Hermelliformia).
Similarly where multiplication has arisen out of the subdivision of a unit into smaller units, we can again subdivide these smaller units.
The essential difficulty has been that none of them could subdivide the rapidly solidifying charge into the small balls which the workman dexterously forms by hand, and that if the charge is not thus subdivided but drawn as a single ball, the cinder cannot be squeezed out of it thoroughly enough.
It is, however, impossible to subdivide the Sierra Madre into a northern and a volcanic chain; for the volcanoes are isolated by stretches of comparatively low countr y; at least thirteen considerable streams flow down between them, from the main watershed to the sea.
The Hydromedusae are distinguished from the Scyphozoa chiefly by negative characters; they have no stomodaeum, that is, no ingrowth of ectoderm at the mouth to form an oesophagus; they have no mesenteries (radiating partitions) which incompletely subdivide the coelenteron; and they have no concentration of digestive cells into special organs.