THE DIGESTIVE ORGANS OF THE ALCYONARIA. 341 
MESENTERIAL FILAMENTS. 
E. B. Wilson (1884, p. 12) has shown that the dorsal 
mesenterial filaments have almost precisely the same structure 
throughout the Alcyonaria. As they are fully described, and 
their ectodermic origin established by him, it is necessary to 
add but little to his excellent account of these structures. 
Throughout the family these filaments are very long. In 
the siphonozooids of well-marked dimorphic genera they are 
proportionately very much longer and more strongly marked 
than the ventral filaments, and are proportionately less pro- 
nounced in the autozooids. 
In his account of the mesenterial filaments of the Alcyo- 
naria KE. B. Wilson (1884, p. 22) states: 
“There can be no doubt that the compound Alcyonaria are 
derived from solitary forms, which probably possessed eight 
similar filaments, each consisting of an ectodermic circulatory 
part and an entodermic digestive part. As the colony-form- 
ing habit became established, bringing with it the need for 
specialised organs of circulation, a physiological division of 
labour took place among the filaments. In the dorsal pair 
the ectodermic part gradually supplanted the entodermic, 
while the reverse process took place in the other six.” He 
further states that we have no embryological evidence of this, 
but suggests that the portion of the ectoderm (ect.) of the 
stomodzeum which is in immediate continuity with the ventral 
mesenteries probably represents the original ectodermic part 
of the ventral filament. 
A study of vertical sections of the stomodzum shows (fig. 
8) this ectodermal tissue (ect.) to be the ectoderm of the 
lower end of the stomodzeum which has become fused with 
the mesenteries. Between the mesenteries the ectoderm 
becomes thinner towards the free edge, and is identical in 
every sense with that portion which has fused with the 
mesenteries. 
