Hyatt.}] 116 [March 5, 



The absence of ineurrent pores in the Hydrozoa in connection 

 with the gastric cavity, and the apparently secondary origin 

 of the pores in connection with the coelomic sacs of many 

 forms, especially in the Medusae and Ctenophorae, and the 

 absence of the digestive ridges in the cavities of Campanularidae 

 and Sertularidae, the Intaeniolata of Hamann 1 (op. cit., p. 509) 

 are also favorable to our view. On the other hand Hydra, which 

 has no digestive ridges, has an ovum surrounded by a horny case 

 (Kleinenberg) which shows it to be an abbreviated stage of a 

 gonophore and identifies this genus as probably a concentrated 

 form of the marine Taeniolata of Hamann. This view (suggested 

 first by Huxley, Anat. Invert. Am. Ed., 1878, p. 121) we have 

 held for some time to be probable on the basis of general morph- 

 ology, because, as shown by Kleinenberg, the hydra form is devel- 

 oped directly without any free moving planula stage. This is also 

 Haeckel's theoretical position, which views Spongilla and other 

 fresh-water forms as highly specialized on account of their distri- 

 bution and liable to have " quicker" development of the earlier 

 stages. 2 It seems probable, therefore, that the intaeniolate struc- 

 ture may exist not only as a primitive condition of the larva, but 

 arise again at the terminus of taeniolate series in the specialized 

 forms as one of the correllative characteristics of a highly 

 concentrated mode of development. Such degradations are 

 common in fossil Cephalopoda among highly specialized and 

 concentrated forms with local distribution and in the distorted 

 shells of the Steinheim Planorbidae. We have accounted for 

 them by the sweeping away of the progressive characters in the 

 young of the specialized degenerative forms. In the development 

 of Hydra the fleshy septa, the gastrula, hydroplanula, free actinula 



1 Hamann quotes only Koch (Jena. Zeitschr., vol. vn) as havii g seen these ridges 

 but in Parypha crocea (Agass. Contr. Nat. Hist. 1ST. A., vol. in, pi. 23, f. 7), four are 

 figured, and described as " semi-partitions V " in the text. Eucope polystyla, accord- 

 ing to Kowalewsky (Embryol. Monogr. Mus. Comp. Zool., vol. ix, no. 3, pi. 3), and the 

 young scyphistoma stage of Chrysaora, and the young of Actinia, Kowalewsky (ibid, 

 pi. xi), are all described as having vertical septa. 



2 It is possible that this may have some bearing upon the extraordinai'y result of 

 Gotte's research (Zool. Anzeig. 1884, p. 679) in which he has found that the ectodeim 

 disappears and all three of the layers of this sponge are developed from the endoderm. 

 Such an opinion, however, requires very strong support, since it seems at first sight 

 entirely irreconcilable with other observations and implies that the sponges are dis- 

 inct in their earliest stage from all other animals. 



