398 PERCY SLADEN TRUST EXPEDITION 



The ectoderm is a broad layer with long, granular gland cells passing almost to 

 the structureless lamella, regular nematocysts with close spiral and also a few, small, 

 ovoidal nematocysts with loose spiral. The mesenterial filaments are ciliated with granular 

 gland cells and ovoidal nematocysts ; the stomodteal ectoderm is similar. The endoderm 

 of the column, stomodseum and mesenteries is crowded with zooxanthellse, and contains 

 a number of ovoidal nematocysts. Throughout the endoderm are patches of a slightly 

 granular, perhaps yolky substance. 



Loc. Off" Salomon Atoll, Chagos Archipelago, surface. 



4. Larva IV. Zoanthina gardineri, n. sp. (PL 23, figs. 7 — 12.) 



The specimen is somewhat discoidal, r4 mm. long by 2-5 mm. in diameter; the 

 colour is yellow-brown. The aboral part forms almost a shallow saucer into which the 

 base of the oral part accurately fits ; the circular groove is shallower than in the previous 

 larvae. Twelve longitudinal depressions on the surface, corresponding to the attachments 

 of the mesenteries, are found on both parts and to some degree on the oral and aboral 

 surfaces (fig. 7). There is a still deeper depression on the aboral surface than in larva III., 

 this forming an upgrowth within the coelenteron. 



The stomodseum nearly reaches the circular groove and is wider than in the previous 

 larvae, almost quadrilateral ; no ridges are present. The mesenteries reach from pole to 

 pole, but the incomplete ones are very small in the pedicle, where the complete mesenteries 

 almost fill up the coelenteron ; aborally they are of nearly the same radial width. 



The ventral directive mesenteries show peculiarities which are present to some 

 degree in the two lateral pairs of complete mesenteries. On examining serial sections the 

 foUoMang structures are found : 



{a) Towards the base of the stomodseum, near the stomodseal wall, a piece of detached 

 endodermal tissue is seen in each space between the ventral directives and the nearest pair 

 of lateral complete mesenteries. 



(6) These unite with the two complete mesenteries on either side of each space and 

 also with the endoderm of the stomodseal wall (fig. 8). 



(c) On the free edge of each of these webs of tissue an epithelium appears, which 

 lower is found to be due to the growth upwards and outwards over them of the free lower 

 edges of the stomodseum ; structureless lamella is next seen below this epithelium. 



{d) In each space the web continues for some distance attached to the mesenteries 

 on either side. Then their attachments to the lateral mesenteries cease and they appear 

 as outgrowths fi'om the directive mesenteries (figs. 9, 10). 



(e) The stomodseum terminates and the ventral directives are fi:ee with the out- 

 growths, which are now clearly those of the mesenterial filaments, attached to one side of 

 each. 



[f) On the opposite side of each of the directives an enormous thickening of 

 endoderm appears, leaving fissures below as in fig. 11. These later become filled up, but 

 the endoderm of these sides of the mesenteries is always very thick. 



Similar structures are found also in the other spaces between the complete mesenteries. 



