304 Proceedings of Royal Society of Edinburgh. [sess. 
peristomium are formed. The nephridia of the protoccele are absent, 
hnt the mesomeric are present in the early condition of ‘ flame-cell 
organs/ and are usually known as the ‘ head-kidneys.’ The 
mesodermic hands arise in the third segment (peri-anal), and by a 
process, first of growth in length and then of bilateral segmenta- 
tion, they form the coelomic cavities of the adult. 
It is clearly shown in the ontogeny of Polygordius that the 
‘ worm 5 is formed from the larva by an immense growth in length 
of the posterior part ( metamere ), immediately behmd the mesomeric 
nephridia , and by a secondary segmentation of the two mesodermic 
bands filling this part. 
We might expect that the mesoderm of the protomere and 
Fig. 15. — Horizontal section through typical trochophore. 
mesomere (prostomium and peristomium) should arise as separate 
archenteric diverticula from the gut, hut a consideration of the 
precocity of ectoderm and endoderm (compared with mesoderm) 
in pelagic larvae (c/., Tornaria and Bateson’s larva) accounts for 
the condition seen in Polygordius , whilst in Lopadorhynchus the 
whole mesoderm is said to arise in this way.* Therefore the 
typical trochophore larva must be regarded as the ‘larvalised’ 
embodiment of the archi-coelomate type, and, mainly for this 
reason, we must regard the Mollusca as elaborations of the archi- 
coelomate type, in which the mesoderm of the protomere and 
mesomere never forms true coelomic spaces. The continuous epi- 
thelial condition of a coelome is only an intermediate stage in its 
* Kleinenberg, K, Zeitsch. wiss Zool., vol. xliv., 1886. 
