No. 3-] DEVELOPMEXT OF MARIXE SPOXGES. 2>2>o 



of the period, whatever it may be, the larvae become sluggish 

 and lie about on their sides on the bottom of the dish. They 

 then attach, the columnar ectoderm cells are transformed into 

 flat cells, and the body of the larva flattens out into a round 

 cake-like mass. The distinction between the pigmented and 

 unpigmented portions of the body is entirely lost, the whole 

 surface becoming red. In PI. XXI, Fig. 84, is shown a surface 

 view of recently attached sponge, and in PI. XXII, Fig. 92, one- 

 half of a vertical section through the same. The long spicules, 

 which form a bundle in the unpigmented end of the larva, 

 become scattered irregularly through the body. In attaching, 

 almost all the larvae I have watched have stuck fast by their 

 sides and not by one end. Such a larva just attached is shown 

 in PL XXI, Fig. 85. Its outline still recalls the outline of the 

 swimming larva (Fig. 80), it not yet having assumed the 

 circular shape of Fig. 84. The body is solid; the ectoderm is 

 entirely composed of flat and very thin cells; and the unpig- 

 mented or spicular pole of the larva {sp. p.) can still be 

 identified both by the absence of pigment and the presence 

 of the long spicules. The line of demarcation between the 

 pigmented and unpigmented regions is not a sharp one, as it 

 was in the swimming larva, and the spicules are no longer 

 arranged in a bundle, but have begun to scatter about, though 

 as yet they are still confined to one end of the sponge. 



The attachment may take place obliquely, so as to bring the 

 spicular pole on the upper surface of the metamorphosed larva, 

 though near the periphery. In a few cases I have seen the 

 attachment take place by the non-spicular pole, a little obliquely, 

 to be sure, as is shown in the surface view. Fig. 82. In this 

 larva the spicular pole was pulled in to such an extent, that 

 at first sight it looked like an opening leading into the interior, 

 though as a matter of fact it was nothing of the kind. When 

 the attachment takes place by the end, as in Fig. 82, the 

 spicular pole comes to occupy a more or less central position 

 on the upper surface of the metamorphosed larva. In PI. XXI, 

 Fig. 83, is given a vertical section through a little sponge, 

 which must have attached by the non-spicular pole, for on the 

 upper surface and more or less in the centre is found the 



