of the Echinoderras. 17 



The development of the Echinoderms is, as Prof. Miiller ob- 

 senes, exactly intermediate between the ordinary process of 

 metamorphosis by ecdysis in insects and the so-called " alterna- 

 tion " of the Trematoda and Aphides. 



The phsenomena of alternation, or as we have called it, " zooid 

 development," take place in two ways — by external gemmation 

 and by internal gemmation. 



The former process is confined to the Polj-pes and Ascidians, 

 which form a series leading from the lowest Radiate to the Mol- 

 luscous tvpes. The latter process on the other hand is restricted 

 to the Worms and Echinoderms, which form a series leading 

 from the lowest Radiate to the Annulose types. 



Now in each series three modifications may be detected. The 

 deutero-zooid is developed either — 1. from a complete segment 

 of the protozooid, when it is difficult to say whether the process 

 is one of internal or external gemmation ; or 2. from a small 

 portion of a segment, including a portion of the digestive canal ; 

 or 3. from a small portion of a segment, an entirely new digestive 

 canal being formed. 



The following table will illustrate the relations of these modifi- 

 cations to one another : — 



Zooid Development by 



.^^^ 



( ^ 



External Gemmation. Internal Gemmation. 



„ jj , /Aphides 



3- Salpa tTrematoda. 



.p r Campanularia Echinodermata. 



■ \ Con-nidse, &c. 



1. Cyansea. Taenia. 

 Nais. 



We have hitherto considered the various zooids of each form 

 to be complementary to one another, and all necessary to the 

 perfect manifestation of the individual. 



But the law of " irrelative repetition " long since established 

 elsewhere by Prof. Owen, is illustrated here in the development 

 of zooid forms where they are not necessaiy to the manifestation 

 of the individual. 



In the Echinoderm there is one larval-zooid and one Echino- 

 derm-zooid — the " individual " would be incomplete without 

 either. 



But in the Cyansea the single Scyphistoma-zooid developes 

 perhaps twenty Cyanaea-zooids, any one of which would have been 

 sufficient to complete the individual. 



The development of the hundreds of polj'pes of a Sertularian 

 appear to be referable to a similar law. Nay, the " generation 



Ann.^Mag.N.Huit.Ser.2. Vol.\m. 2 



