ANATOMY AND DEVELOPMENT OF THE ECHINODERMS I 19 



The development of the Echinoderms is, as Prof. Miiller observes, 

 exactly intermediate between the ordinary process of metamorphosis 

 by ecdysis in insects and the so-called "alternation" of the Trematoda 

 and Aphides. 



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

 development," takes place in two ways — by external gemmation and 

 by internal gemmation. 



The former process is confined to the Polypes and Ascidians, which 

 form a series leading from. the lowest Radiate to the Molluscous types. 

 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 — i. from a complete segment of the 

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

 ternal 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 follo\\ing table ^\■ill illustrate the relations of these modifica- 

 tions to one another : — 



Zooid Development by 



External Gemmation. Internal Gemmation. 



3. Salpa /Aphides. 



I Trematoda. 



^ Campanularia Echinodermala. 



I, Corynidae, &c. 



I. C}"aiicea. Taenia. 

 Nais. 



We have hitherto considered the various zooids of each form to be 

 complementary to one another, and all necessary to the perfect mani- 

 festation 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 .necessary to the manifestation of the 

 individual. 



can only " figuvatively " (bildlich) be compared to gemmation, inasmucli as tlic "formative 

 mass" ari,ses independently. 



But since he says immediately afterwards that " the rudiment of the water-vascular 



system, in general, arises before the rudiment of the parietes of the Echinoderm," and since 



he shows elsewhere that the origin of the water-vascular system is by the development of a 



bud-Uke process inwards — the process may, we think, be called gemmation in much more 



han a figurative sense. 



