. Elements of comparative anatomy. Anatomy, Comparative. 98 COMPARATIVE ANATOMY. becoming greatly increased into a wide cavity, the air sac occupies tlie greatest part of the stem, and thus forms the largest portion of the colony, the separate pieces of which look like appendages placed on one side of the bladder. This character is greatly developed in the Physalidas, and is accompanied by the shortening of the stem. Another condition obtains in the Velellidas, where the air sac is placed on the end of the greatly shortened trunk, and is developed by lateral extension into a disc, the cartilag
. Elements of comparative anatomy. Anatomy, Comparative. 98 COMPARATIVE ANATOMY. becoming greatly increased into a wide cavity, the air sac occupies tlie greatest part of the stem, and thus forms the largest portion of the colony, the separate pieces of which look like appendages placed on one side of the bladder. This character is greatly developed in the Physalidas, and is accompanied by the shortening of the stem. Another condition obtains in the Velellidas, where the air sac is placed on the end of the greatly shortened trunk, and is developed by lateral extension into a disc, the cartilaginous firm walls of which divide the internal cavity into numerous chambers by forming walls of partition. In the earliest stages of development the air sac is simple in these forms also. In Porpita, the disc remains flat and circular; in Velella it is produced into a diagonal vertical crest, into which the air spaces of the disc are not continued. The concentri- cally-arranged chamber-spaces of the air sac in Velella are connected by apertures. They open to the exterior by a number of holes placed on the surface. In Porpita, fine air passages, in the form of canals, pass off from the inferior surface of the air sac, and enter into and branch in the portion of the stem, which carries the nutritive individuals. § 76. The Thecomedusa) are polypoid Ccelenterata provided with a test, and are allied to the Hydriformes, although in organisation they resemble Medusas; they are indeed, intermediate between these two groupsj for they are representatives of forms which are closely allied to the larva) of the Discophora. This larval form (Scyphostoma) seems to be more highly organised than most of the Hydroid- Polyps; it presents, indeed, points of connection with only a few of them (Cory- morpha). It is developed, just like the Hydroid-Polyps, from a planula, which is at first free, and which after- wards becomes fixed. But the fundamental form of the body resembles not only that of man
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