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Proceedings of Poyed Society of Edinburgh. [sess. 
like diverticulum, which is invaginated from the body wall, and 
later evaginated to form the greater part of the body of the adult 
Phoronis, is strengthened by a vacuolisation of certain of the hypo- 
blast cells, which present a structure identical with that of the paired 
notochord in the collar region, hut do not form a diverticulum. 
They thus present the most primitive form of hypoblastic 
skeleton, the cells being metamorphosed in situ. The areas of 
notochordal tissue in Actinotrocha present a much more diffuse 
and de facto primitive condition than is found in the other 
Chordata. Their subsequent fate is unknown, hut they are 
certainly not present in the adult.* 
In the former paper was instituted a comparison between the 
structure of Balanoglossus and Phoronis , the points of similarity 
being well marked, provided due allowance be made for the 
different environment of these two animals and its effect in 
each case upon their anatomy. 
As is to be expected, a comparison between the larval forms 
Actinotrocha and Tornaria reveals even closer harmony of structure ; 
the resemblances between these two forms point to an extremely 
close affinity between them. It must be sufficient here to enumer- 
ate the leading characters in common : — 
A large overhanging pre-oral lobe. 
An apical plate, with, in some cases, two eyespots. 
A pre-oral ciliated ring. 
A post-oral ring, complicated (in some Tornaria) by tentacular 
protrusions of the body. 
A peri-anal ring of cilia. 
Gut with oesophagus, stomach, hind -gut, and terminal anus. 
Mesoderm forming five coelomic pouches, one pre-oral and 
two pairs post-oral. 
Dorsal and ventral blood-vessels derived from the hsemocoelic 
space, connected by a collar-ring. 
The only important character in Tornaria which, as far as is 
known, has no homologue in Actinotrocha is the proboscis pore. 
Summing up, we may say that a study of the structure of 
* I hope to shew later that there are paired lateral gut diverticula in the 
Asterid larva, which may be regarded as vestigial hornologues of those here 
described. 
