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THE ENTEROPNEUSTA. 661is almost invariably found where the lateral septum joins the basement-membrane at thetip of the pleura. Occasionally an ingrowth from the ectoderm of the inner surface of thepleura is to be seen, but this is rare. The irregular occurrence of the ectodermal connectionsand their somewhat attenuated condition seems to indicate that they are in process ofdisappearance. And this supposition is borne out by the fact that in somewhat olderspecimens the gonads are devoid of an}- ectodermal connection.(2) Var. sa.vicola. Though all the specimens of this variety were of fail- size the gonadsin all are in a very backward state of development. The gonads are in all cases youngand in all cases possess a more or less well-marked ectodermal connection. This connectionis relatively wider in the }-ounger gonads (cf. PI. XL. figs. 41 and 42). A still youngerstage of an ectodermal ingrowth at the tip of a genital pleura is shewn on PI. XXXIX.fig. 31. There can be little question but that this is of the nature of an ingrowth, andindeed rounded cells may be seen in it, which are not to be found in other parts of theectoderm, and which are probably to be regarded as primitive genital cells. The mainingrowth is that at the tip of the pleura as in var. laccadivensin, but the accessory ingrowthson the inner surface of the pleurae are mvxch more numerous in saxicola, a fact probablycon-elated with the larger size attained by the animal for a similar stage of gonidial development.Unfortunately none of the four specimens of this variety possessed older gonads.(3) Var. inuscula. In a specimen with immature and somewhat small gonads an ectodermalconnection is generally wanting. It may be present here and there. In an olderspecimen in which spermatocytes could be recognized in the gonads (PI. XL. fig. 37) theectodermal connections are present on almost all the gonads.(4) Var. maldivensis. This variety is characterized by often having long slender genitalducts. The}' are found connected with tairl}- well-developed gonads and in some cases Ihave been able to make out their double origin, in part from an ectodermal ingrowth andin part from an outgrowth from the gonad. In one or two instances the two portions hadnot eflPected a junction, a condition sujjporting the view that the true genital ducts arelate in making their appearance. It has already been noticed that the ectodermal ingrowthsnear the tijj of the pleurae always occur directly over the point where the lateral septumjoins the basement-membrane and pass down between its lamellae. On the other hand theducts in connection with the gonads nearest the tip of the pleurae may be entirely independentof the lateral septum (PI. XXXIX. fig. 27).All the above observations are in harmony with the view that the gonads are (inPt. flava at any rate) derived from ectodermal ingrowths, that they subsequently lose theirconnection with the ectoderm, but regain it later when the genital ducts are established.Further, on this view we can explain the latei-al septum of the Ptychoderidae, regarding itas having in the first place been brought about by the basement-membrane being carriedin by the primitive ectodermal ingi-owths at the tip of the pleurae (at that time probablyvery much smaller). On this view the lateral septum should be four-layered since thebasement-membrane consists already of two layers enclosing blood spaces. That four potentiallayers are present is shewn b}' the fiict that the apparently single layer round the gonadis in reality a double one since blood spaces frequently occur in it (PI. XL. fig. 41). Theca\'ity of the gonad is therefore quite distinct from the blood spaces, which militates againstSpengel's view of the nature of the gonad.The gonads of Ptychodera jJava are characterized by the jjresence of peculiar bodiesG. II. 85