780 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
sheet which nearly encircles the body, but is interrupted at certain points where the 
longitudinal fibres are situated. As seen in surface view (figs. 160, 161 ¢.m.) they 
appear as delicate transparent fibres which are quite disconnected from one another. 
The sheet of circular fibres extends at first no further forwards than the first pair of 
buds ; but as development proceeds the fibres extend forwards to the oral extremity. 
In the tentacles they seem never to be developed. 
The longitudinal fibres differ from the circular in being at first arranged in definite 
tracts. These correspond in part with the septa, a narrow band of fibres following 
the line of attachment of each of the radial septa, and of the peduncular septum on 
each side of the body ; these may be termed the septal tracts. Besides these there 
are two median tracts extending forwards, above and below, from the posterior 
extremity of the body. The dorsal median tract extends forwards nearly to the 
first pair of buds and there terminates. The ventral median tract bifurcates at its 
anterior extremity, and the two branches become continuous with the tracts of the 
ventral septa (see fig. 176 and p. 765). This arrangement of the longitudinal muscles 
is strongly bilateral, the median plane corresponding with the dorso-ventral axis of 
the body. From these primitive tracts the longitudinal fibres gradually extend 
laterally until they form an unbroken sheet lying within the circular fibres. They 
also extend forwards into the tentacles, and out towards their tips. 
In transverse sections the longitudinal fibres appear as small dark spots lying in 
the basal part of the entoderm just within, and in contact with the supporting 
lamella. The entoderm cells covering the median tracts always show a fan-shaped 
arrangement, which is especially marked in younger stages, while the tract is still 
very narrow, as shown in fig. 162 (dorsal tract). Fig. 164 represents the ventral 
tract of a young specimen, and fig. 163 the same tract of an older individual. 
The septal tracts (fig. 165) are divided into two parts by the lamella of the septum. 
In early stages these two parts lie at the base of the septum; but as development 
proceeds the fibres on the ventral side gradually extend out into the septum, until at 
length they cover a broad tract on the ventral face of the lamella. They also extend 
for some distance out upon the ventral face of the peduncular septum (fig. 167). (The 
presence of longitudinal muscles in the lateral portions of the peduncular septum is 
mentioned by KouiiKER, ‘ Pennatuliden,’ p. 274.) These muscles form the retractors 
of the polyp, which have therefore the arrangement characteristic of all the Alcyo- 
narian polyps which have thus far been examined, (Cf K6iiiKkeEr, Ersen, Linpaut, 
MoseEey, and HaAacke.) 
The circular muscles can scarcely be seen in transverse sections, but are here and 
there visible, as in fig. 169. In longitudinal sections they are plainly visible (fig. 168) 
as a series of dark spots within the supporting lamella. They are somewhat irregu- 
larly placed, and are not grouped in definite tracts. 
