156 ERNST HAECKEL. 



the Ctenophora and corals (PI. VII, fig. 2). In the class of the 

 worms the same gastrula (the so-called '* iiif'usorian-like 

 emhvyo ") occurs sometimes in exactly the same, sometimes 

 in a more or less modified form, in the flat-worms (Turhel- 

 laria, PI. VII, fig. 3, and Trematoda) ; in the roundworms 

 (Nematoda, Sagitta) ; in the Bryozoa (Polyzoa) and Tunicata 

 (Ascidia, PI. Vlljfig. 4); in theGephyrea and Annelida (Pho- 

 ronis, Euaxes, Lumbricus, Chsetopodai). In the class of the 

 Echinodermata the gastrula appears to be very widely distri- 

 buted in allfourdivisions, especially in the Asteridae and Holo- 

 thuridea" (PI. VII, fig. 6). In the class Arthropoda the gastrula 

 is, indeed, nowhere any longer completely preserved in its 

 original sim|)le form, but it is very easy to reduce the earliest 

 embryonic forms of the nauplius (as the common root-form 

 of the Crustacea), and of many of the lower tracheal breath- 

 ing forms to the gastrula^ (PI. VII, fig. 7). In the class 

 Mollusca, the gastrula appears to be widely distributed, 

 especially in the groups of Mussels and Snails, and probably 

 also in the Spirobranchia ; among the snails it has been first 

 observed iu Limnaeus* (PI. VII, fig. 5). Lastly, in the class 

 Vertebrata the orginal gastrula form is only fully preserved in 

 the Acrania (Amphioxus, PI. VII, fig. 8). Nevertheless the 

 continuity which exists between the ontogenesis of the Am- 

 phioxus and of the other Vertebrata leaves no doubt remain- 

 ing that the ancestors of the latter have passed through the 

 gastrula form in earlier periods of the earth's history at the 

 commencement of their ontogenesis.^ 



The phenomenon, that the gastrula recurs in the same 



' On the gastrula of the worms, the works of Kowalevsky are especially 

 to be cousulted — 'Memoires de I'Acadeinie de St. Petersburg,' torn, x, No. 

 15 (18G7) ; torn, xvi, No. 12 (1871) ; his ontogenesis of Phoronis, of the 

 Ascidia, and his embryological studies on worms and Arthropoda. 



2 The gastrula of the Echinodermata are made intelligible to us by the 

 figures of Johannes Miiller, of Alexander Agassiz (' Embryology of the 

 Starfish,' PL I, fig. 25 — 28), and by Kowalevsky (' Ontogenesis of the 

 Holothuria'). 



' That the ancestors of the Arthropoda must also have developed them- 

 selves from the gastrula, is clearly proved by the comparison of their 

 simplest and earliest immature condition with the gastrula of the worms. 

 Compare especially the works of Edouard van Beneden and Bessels ' On 

 the ontogenesis of the Crustacea,' and of Weismann ' On the ontogenesis 

 of Insects.' 



* E. Kay Lankester has described the gastrula of the Mollusca in a 

 recent paper ('Annals and Mag. of Natural History,' Eeb., 1873, pp. 86, 

 87). In many mussels and snails it is developed in exactly the same 

 manner as iu the Zoophytes, worms, Echinodermata, Amphioxus, &c. 



* The gastrula of the Vertebrata, which only now persists in Amphioxus 

 has been made known to us by Kowalevsky in his ontogenesis of this oldest 

 Vertebrate animal (1. c, PI. I, figs. 16, 17). 



