LARVAL FORMS : THEIR NATURE, ORIGIN AND AFFINITIES. 407 
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 praeoral lobe, and in the pre- 
sence 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 proto- 
type 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, and their 
nervous ring is probably directly derived from that 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 
Coelenterate larvse. 
5. The points of similarity between Tornaria and (1) the 
Trochosphere and ( 2 ) the Echinoderm larvse are probably 
adaptive in the one case or the other ; and, while there is no 
difficulty in believing that those with the Trochosphere are 
adaptive, the presence of a water-vascular vesicle with a 
dorsal pore renders probable a real affinity with Echinoderm 
larvge. 
6. 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. 
The majority of these conclusions are undoubtedly of a 
highly speculative character, but while they cannot be 
regarded as part of our stock of embryological knowledge, 
they may, nevertheless, serve to indicate an important line 
for continued embryological research. A thorough histo- 
logical investigation of the larval forms dealt with in this 
essay will be likely to lead to valuable results. 
