74 Miscellaneous. 



stomial aud metastomial cavities are placed in communication ; 2. 

 The ganglionic masses of the nervous system are formed above the 

 lateral organs, between them and the cephalic masses. Soon the 

 whole, formed by the union of the cephalic masses, the nervous 

 system, and the lateral organs, becomes fashioned into a coherent 

 mass, which is the head of the adult. Here the head corresponds 

 exactly to the prostomium, and the body to the metastomium ; the 

 oesophagus, therefore, is situated entirely outside the head. 



In the Enopla the ganglionic masses, instead of being formed above 

 the lateral organs, are formed beloiv them, between the latter and the 

 muscular laminae of the body- wall. Combined with the lateral organs 

 and the oesophagus they constitute a new septum, which continues 

 to keep the two cavities distinct. Subsequently the whole of these 

 different parts (cephalic masses, nervous ganglia, lateral organs) are 

 condensed, as in the Anopla, into a coherent mass, which represents 

 the head. Here the head includes, besides the prostomium, a part 

 of the metastomium, with the portion of the oesophagus that is con- 

 tained in the latter. 



The conformity just indicated in the results of the first embryonic 

 processes, and the constant presence of a common stage, demonstrate 

 the general unity of the plan of development. It remains to see 

 whether there exists a gradual passage between the different modes 

 of production of that stage. 



Let us take for example two embryogenies belonging to the two 

 fundamental types of development, namely Lineus communis and 

 Amphiporus lactijlorens. 



The chief phenomena of the embryogeny of Amphiporus are: — 1, 

 the accumulation of deutoplasmic elements at the inner part of the 

 segmentation-spheres, arranged radiately, and the formation of an 

 inner nucleus of deutoplasma, which represents the middle lamella ; 

 2, the invagination at one point of the superficial layer (r/astnda) ; 

 the invaginated cellular mass (endoderm) penetrates into the interior 

 and coalesces with the nucleus of deutoplasm ; we then obtain a 

 stage formed of a superficial layer and of an interior mass ; the super- 

 ficial layer is the exoderm, and the interior mass represents the 

 union of the middle and inferior lamellae. The interior mass then 

 undergoes a direct differentiation into muscixlature (which rapidly 

 acquires its characteristic arrangement) and endodermie fatty ele- 

 ments (which collect in the metastomial cavity). The essential fact 

 of the development is reduced, indeed, to the formation of the mus- 

 culature by the simple differentiation of a continuous lamella. 



Lineus communis presents generally in its embryogeny an identi- 

 cal course ; only the musculature is formed by the coalescence of three 

 primitively distinct pairs of rudiments. As regards the two primitive 

 lamellae there is complete concordance in both cases ; the only dif- 

 ference, therefore, in the two most dissimilar types consists in the 

 mode of formation of the mesoderm, the subsequent evolution of 

 this lamella being the same on both sides. From this we may con- 

 clude that there exists a perfect continuity between the different 

 modes of jjroduction of the common stage. 



