386 GENERAL CONCLUSIONS. 



part of the nervous ring gave rise (?) to the supra-oesophageal 

 ganglia, and the optic organs connected with them ; while the 

 posterior part of the nerve-ring formed (?) the ventral nerve-cords. 

 The body cavity was developed from two of the primitive 

 alimentary diverticula. 



The usual view that radiate forms have become bilateral by 

 the elongation of the aboral dome into the trunk is probably 

 erroneous. 



4. Pilidium is the larval form which most nearly reproduces 

 the characters of the larval prototype in the course of its 

 conversion into a bilateral form. 



5. The Trochosphere is a completely differentiated bilateral 

 form, in which an anus has become developed. The praeoral 

 ciliated ring of the Trochosphere is probably directly derived 

 from the ciliated ring of Pilidium, which is itself the original ring 

 of the prototype of all these larval forms. 



6. Echinoderm larvae, in the absence of a nerve-ganglion or 

 special organs of sense on the prasoral lobe, and in the presence 

 of alimentary diverticula, which give rise to the body cavity, 

 retain some characters of the prototype larva which have been 

 lost in Pilidium. The ciliated ring of Echinoderm larvae is 

 probably derived directly from that of the prototype by the 

 formation of an anus on the dorsal side of the ring. The anus 

 was very probably originally situated at the aboral apex. 



Adult Echinoderms have probably retained the radial sym- 

 metry of the forms from which they are descended, their nervous 

 ring being directly derived from the circular nervous ring of their 

 ancestors. They have not, as is usually supposed, secondarily 

 acquired their radial symmetry. The bilateral symmetry of the 

 larva is, on this view, secondary, like that of so many Ccelenterate 

 larvae. 



7. The points of similarity between Tornaria and (i) the 

 Trochosphere and (2) the Echinoderm larvae are probably 

 adaptive in the one case or the other ; and, while there is no 

 difficulty in believing that those to the Trochosphere are 

 adaptive, the presence of a water-vascular vesicle with a dorsal 

 pore renders probable a real affinity with Echinoderm larvse. 



8. It is not possible in the present state of our knowledge 

 to decide how far the resemblances between Actinotrocha and 

 Echinoderm larvae are adaptive or primary. 



