1884-] 117 [Hyatt. 



and colonial form, have all suffered or been more or less suppressed 

 by concentration, so that the degenerate hydraform has a direct 

 mode of development from an inclosed planula by delamination. 

 This is, also, approximately similar to the views of Balfour, 

 though he makes his comparisons from the stand point of natural 

 selection (Comp. EmbryoL, vol. i, pp. 127, 151.). A comparison 

 of Ciamician's figures of the development of Tubulariamesembry- 

 anthema (Zeitschr. Wissen. Zool., vol. xxxn, pi. 18-19), with 

 Kleinenberg's famous paper on the development of Hydra, ena- 

 bles one to see that the epibolic gastrula of the Tubularidae 

 belongs to a very early stage of the ovum, and must have in some 

 way become unnecessary to the abbreviated delamination by 

 which the inner layer is formed in the young of Hydra. 



This result has not in the least surprised us since we have been 

 constantly in the habit of finding homoplastic characteristics aris- 

 ing in similar parts of animals as closely related as two genera in 

 the same family or forms in different families or orders of fossil 

 Cephalopoda. So far, therefore, as the sponges and Actinozoa are 

 concerned, purely mechanical reasons appear to be applicable to 

 the origin of the diverticula. In the one case the pores and growth 

 of the mesenchyme, in the other the ingrowth of fleshy septa. The 

 phenomena in both cases, however, take place as a direct result 

 of the extra growth of specialized assimilative cells and mem- 

 branes, and the correlations seem to point to excess of nutri- 

 ment as the primary cause ; the excess being localized in the am- 

 pullae and mesenchyme of the Porifera, and in the taeniolate 

 ridges, and endodermic mesenterial filaments of the Coelenterata. 

 Paleontologists have been of late rapidly bringing forward testi- 

 mony, as has Prof. Cope in several essays, that " homologous " 

 forms may occur in parallel series which from this point of view 

 can be called " heterologous" in their own series. Homology in 

 other words signifies simply identity of position in relation to 

 other parts, or other forms, and probably a common origin for the 

 different series in which they occur ; but these homologous parts 

 and forms may very often be purely homoplastic in origin. 



The development of the actinostome occurs late in the life of 

 the Actinozoon (see EmbryoL Monogr. Mus. Comp. Zool., vol. ix, 

 no. 3), and there is no longer any doubt that this is an invagina- 

 tion of all the layers of the body at the rim of the blastopore. 



