ORGANS OF THE SPAT 69 



end outwards. Each is composed of a surrounding row of epithelial cells 

 close together, so as to leave no or scarcely an^ lumen, with sometimes a 

 few small connective-tissue cells. In spat of 60 units length some of the 

 filaments have become very broad from side to side, and their anterior 

 and posterior walls approach each other in the middle as if to constrict 

 into outer and inner halves, connected by a narrow transverse bridge. The 

 outer and inner edges are similarily modified, and the parts each side of 

 the constriction are formed alike. Spat of a length of 67 have some of the 

 filaments of the left gill constricted in two, and it is possible to recognize 

 the frontal epithelium and respiratory cilia, the supporting rods and 

 tissues and small lacunae. In spat of 110 units this is carried so far that 

 the gill is imperfectly split into outer and inner lamellsp, the outer and inner 

 halves of each filament being widely separated above, but remaining con- 

 nected below (Plate VI, fig. 26). Anteriorly the inner lamelloe of the two 

 gills approach each other, towards the median ventral line of the abdomen, 

 where they are attached, but behind this they unite, separating off a supra- 

 branchial chamber above from an infra-branchial chamber below, the 

 supra-branchial chamber dipping down between the two lamellae of each 

 gill. Posteriorly this is extended by the formation of new filaments, 

 which push the older ones forwards along the growing abdomen, each new 

 filament developing in the same way as the preceding, so that the lamellae 

 of a gill are united not only along the ventral edge but behind as well. 

 Water from the infra-branchial chamber passes through the slits between 

 the filaments, into the inter-lamellar spaces, and up into the supra- 

 branchial chamber, from which it can escape posteriorly. These slits, 

 with the growth of the gills, come to be interrupted by inter-filamentar 

 junctions, that divide them into ostia. The spat of length 60 already has 

 a row of such junctions, connecting the tips of the filaments of the left 

 gill. In some cases these appear to have persisted from the first. In a 

 similar manner, during the splitting of the filaments in the formation of 

 outer and inner lamellae, the separation is often incomplete, leaving bridges 

 of inter-lamellar junctions. Inter-filamentar and inner-lamellar junctions 

 help to support and hold in place the delicate net-work of the increasingly 

 large and complex branchial apparatus. With growth in length the 

 axes come to curve downwards and backwards, as if doubling round the 

 great adductor muscle, so that the anterior filaments project forwards, but 

 the posterior ones ventralwards, and vertical transverse sections of the 

 spat cut the anterior filaments transversely, but the posterior ones longi- 

 tudinally. When the right and left outer gill-leaves develop they run 

 through a similar history, the upper edges of their inner lamellae becoming 

 connected with the corresponding outer edges of those already in existence 

 and the upper edges of the outer lamellae becoming connected with the 

 mantle. 



