ALTERNATION OF GENERATIONS. 141 



germ-mass without any breach of structural continuity — 

 and compare it to the Tsenia-head of a Cestoid worm, or the 

 zoophytic phase of polype-life ; in which case the chain of 

 sexual Salpa? would represent simply an aggregation of 

 generative structures, provided (like the proglottides of the 

 Twnia or the medusoids of the polype) with accessory 

 organs of nutrition and locomotion. Either of these com- 

 parisons perhaps might stand of itself, but the weight of 

 analogy is much in favour of the latter ; the parallel between 

 the chain of sexual ^alpce, tlie pile of medusoids, the 

 jointed " body" of the Tapeworm, and tlie caudal train of 

 some Annelida — all alike made up of segments furnished 

 with proper sex-organs, and maturing true ova — is too dis- 

 tinct to be overlooked. In this direction accordingly it is 

 that we find those naturalists who have made this class 

 their especial study are the most inclined to look for illus- 

 trations. Mr. Huxley, in particular, is very decided in 

 favour of this view. In his paper in the Philosophical 

 Transactions on the Anatomy of Salpa and Pyrosoma, he 

 characterizes the aggregated zooids as merely highly indivi- 

 dualized generative organs,* and in a lecture at the Royal 

 Institution he ranks amono; their homolonues the medusiform 

 zooids of Physalia and Velella, the medusiform organs of 

 Diphyes and Tubidaria, and the egg-capsules of Hydra.'\ 



In this view, of course, we cannot recognize any example 

 of protomorphic alternation among the Salpcu. It is by no 

 means clear, however, that all the derivative Tunicata are 

 on a footing in this respect with the catenated Salpce. Ac- 

 cording to MM. Lowig and Kolliker, the stellate clusters 

 of the Botryllida are formed by a sort of fissiparous divi- 

 sion of the original germ. In this case their formation 

 would be parallel to the protomorphic gemmation of the 

 Trematoda, or as Dr. Carpenter remarks, " to the free gem- 



* Phil. Transac. for 1851, p. 5 11. 

 t Ann. Nat. Hist., 2d Ser., IX., p. 506. 



