REPORT ON THE MONAXONIDA. U 



mammiformis, nol:)is. In the particular stage referred to the larva is more or less 

 spherical in form, and composed of an outer (? single) layer of small, ciliated, ectodermal 

 cells, and a central mass of much vacuolated tissue. The central mass of tissue has 

 probably been a good deal affected by the action of the spirit ; it appears now to consist 

 of strings of plasma (apparently composed largely of stellate cells) containing numerous 

 spherical nuclei and nucleoli. Probably in the living embryo this central mass was 

 composed of a continuous, structureless, gelatinous matrix with embedded stellate cells, 

 like the ground substance of the adult sponge, but more delicate. Conspicuous in it are 

 numerous stylote and anisochelate spicules, arranged apparently without any order. The 

 palmate anisochelae are more numerous and appear to have reached a higher state of 

 development than the styli. They average about 0'05 mm. in length as against 0"072 

 mm. in the adult sponge. The styli on the other hand measure only about 0'34 by 0'005 

 mm., as against 1"0 by 0"019 mm. in the adult. 



The entire embryo measures up to nearly I'O mm. in diameter, and is enclosed in a 

 membranous capsule. The ectodermal layer in three specimens which we have examined 

 at about this stage exhibits foldings or invaginations which may or may not be due to 

 the action of the spirit. 



In a younger embryo of the same sponge we have been able to trace the development 

 of the palmate anisochelse, as recorded on p. xx. 



In Esperella hiserialis the embryos found in the spicular axis of the sponge agree 

 essentially with those just described for Esperella mammiformis, i.e., they consist of an 

 irregular sphere of small ectodermal cells enclosing a central mass of tissue with numerous 

 spicules. In one embryo, howcA'-er, the ectodermal layer appears to be absent from 

 one pole, at which the central mass of tissue -comes to the surface. The embryos are 

 again enclosed in membranous capsules. 



In Myxilla nohilis we have found numerous embryos in various stages of develop- 

 ment, amongst which a stage corresponding with that first described is again common. 

 The embryo, about 0"24 mm. in diameter, consists of a sphere of ectoderm, which may 

 be invaginated in many places, composed of small, prismatic (?) cells, enclosing a mass of 

 tissue, in this case rather compact, containing numerous megaselera and microsclera. It 

 is important to observe that the styli of the embryo, which are usuall}' arranged in a single 

 dense sheaf with their bases all together at one side of the embryo and their apices pro- 

 jecting into the centre, are all entirely spined and resemble the echinating styli of the adult 

 sponge, and not the large styli, which in the adult are entirely smooth or only very slightly 

 spined at the base [vide p. 141). They are straight, and gradually and sharply pointed, 

 sometimes with distinct heads, and measure about O'll by 0-005 mm. The chelae appear 

 to develop before the styli. The characteristic arrangement of the megaselera in the 

 embryo appears to have already been noted by Carter in an embryo of a siliceous sponge.^ 



1 Ann. and Mag. Nat. Hist, ser. 4, vol. xiv. p. 333, pi. xxii. fig. 28, e. 



