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 groups j 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 many Hydroid- 

 Polyps, but their Medusa 

 stage also, for two equiva- 

 lent secondary axes cross 

 th e primary one. The organ s 

 are arranged by fours, so 

 that four antimeres can be distinguished in the body. Medusa? are 

 budded off from this polyp-form, but gemmation does not, as in the 

 Hydroid-Polyps, take place at the side, but at the end. The terminal 



=C 



Fig. 34. Young stages of Aurelia aurita. 

 1 Planula-form, attaching itself. 2, 3 Passage 

 into the Polyp-form. <1 Commencement of the 

 formation of metameres. 5 Continued forma- 

 tion of mctarnercs (Strobila) and the differen- 

 tiation of them (after M. Sars). 



