44 
W, F. PUECELL. 
unusually distended and the operculum more upright, I find 
the septa placed almost horizontally (e. g. in a specimen of 
Nephila from Senegal). 
VI. The Development op the Abdominal Longitudinal 
Muscles and theik Tendons. 
In their earliest stage the coelomic sacs of the eighth to 
eleventh segments each protrude an evaginated portion of 
their somatic wall into a provisional appendage, completely 
lining the cavity of the latter (fig\ 4). At this stage {St. I) 
the cells of the somatic wall of the sac are cylindrical, and 
much higher than those of the splanchnic wall. 
At the time when the first pulmonary furrows begin to appear 
(stage 2) the intra-appendicular portion becomes partially 
cut off from the main coelomic sac by an infolding of its wall 
along the medial basal edge of the appendage (fig. 1). The 
infolded layer grows in a lateral direction halfway across the 
cavity of the appendage, converting the medial half of the 
intra-appendicular coelom into a short tube. Each of these 
segmental tubes lies in a transverse plane, is blind at the 
medial end, and opens laterally into the coelomic sac.^ 
In a longitudinal section through the lateral region of the 
appendages {ah. ai)p. 1-3, fig. 5a) the main coelomic cavity is 
seen to be continuous with the intra-appendicular portion, 
whereas the latter portion in a more medial section of the 
same series (fig. 5) appears separated from the main coelom, 
the segmental tubes {seg. t. 3-10), of course, being seen in 
cross section. If this latter section be compared with a 
corresponding section of a later stage (fig. b, stage with five 
pulmonary furrows), it will be noticed that the portion of the 
somatopleura which formed the inner layer of the above- 
‘ I have sliown elsewhere ('95) that the segmental tubes of the eighth 
or pulmonary segment become the genital ducts. In fig. 23 b (adult 
male) the mesodermal part of the genital duct [mes. fj. d.), derived from 
the segmental tubes, is seen sharj^ly differentiated from the ectodermal 
portion {ec. <j. d.). 
