THE ENTEROPNEUSTA. 661 
is almost invariably found where the lateral septum joins the basement-membrane at the 
tip of the pleura. Occasionally an ingrowth from the ectoderm of the inner surface of the 
pleura is to be seen, but this is rare. The irregular occurrence of the ectodermal connections 
and their somewhat attenuated condition seems to indicate that they are in process of 
disappearance. And this supposition is borne out by the fact that in somewhat older 
specimens the gonads are devoid of any ectodermal connection. 
(2) Var. sawicola. Though all the specimens of this variety were of fair size the gonads 
in all are in a very backward state of development. The gonads are in all cases young 
and in all cases possess a more or less well-marked ectodermal connection. This connection 
is relatively wider in the younger gonads (cf. Pl. XL. figs. 41 and 42). A still younger 
stage of an ectodermal ingrowth at the tip of a genital pleura is shewn on Pl. XXXIX. 
fig. 31. There can be little question but that this is of the nature of an ingrowth, and 
indeed rounded cells may be seen in it, which are not to be found in other parts of the 
ectoderm, and which are probably to be regarded as primitive genital cells. The main 
ingrowth is that at the tip of the pleura as in var. laccadivensis, but the accessory ingrowths 
on the inner surface of the pleurae are much more numerous in saaicola, a fact probably 
correlated with the larger size attained by the animal for a similar stage of gonidial develop- 
ment. Unfortunately none of the four specimens of this variety possessed older gonads. 
(3) Var. muscula. In a specimen with immature and somewhat small gonads an-ecto- 
dermal connection is generally wanting. It may be present here and there. In an older 
specimen in which spermatocytes could be recognized in the gonads (Pl. XL. fig. 37) the 
ectodermal connections are present on almost all the gonads. 
(4) Var. maldivensis. This variety is characterized by often having long slender genital 
ducts. They are found connected with fairly well-developed gonads and in some cases I 
have been able to make out their double origin, in part from an ectodermal ingrowth and 
in part from an outgrowth from the gonad. In one or two instances the two portions had 
not effected a junction, a condition supporting the view that the true genital ducts are 
late in making their appearance. It has already been noticed that the ectodermal ingrowths 
near the tip of the pleurae always occur directly over the point where the lateral septum 
joins the basement-membrane and pass down between its lamellae. On the other hand the 
ducts in connection with the gonads nearest the tip of the pleurae may be entirely inde- 
pendent of the lateral septum (Pl. XX XIX. fig. 27). 
All the above observations are in harmony with the view that the gonads are (in 
Pt. flava at any rate) derived from ectodermal ingrowths, that they subsequently lose their 
connection with the ectoderm, but regain it later when the genital ducts are established. 
Further, on this view we can explain the lateral septum of the Ptychoderidae, regarding it 
as having in the first place been brought about by the basement-membrane being carried 
in by the primitive ectodermal ingrowths at the tip of the pleurae (at that time probably 
very much smaller). On this view the lateral septum should be four-layered since the 
basement-membrane consists already of two layers enclosing blood spaces. That four potential 
layers are present is shewn by the fact that the apparently single layer round the gonad 
is in reality a double one since blood spaces frequently occur in it (Pl. XL. fig. 41). The 
cavity of the gonad is therefore quite distinct from the blood EES, which militates against 
Spengel’s view of the nature of the gonad. 
The gonads of Ptychodera flava are characterized by the presence of peculiar bodies 
G. IL. 85 
