JOUSSEAUMIA. 259 



Although I made a careful search, I was unable to find any ova or embryos in the 

 supra-branchial chamber. It does not necessarily follow that Jousseaumia is not 

 incubatory, and that the ova are not fertilised, and undergo the earlier stages of 

 development in the branchial cavity, for as I have pointed out, there is probably a 

 seasonal alternation of sexual maturity, and the specimens at my disposal were mostly 

 in the male condition. Even those which had advanced beyond this stage and contained 

 numerous ova in the gonads, did not give evidence of complete female maturity. In 

 one specimen of Heteropsammia I found, in the Aspidosiphon chamber, a number of 

 small ovoid ova, each surrounded by a thick radially striated egg-membrane, but I 

 have no evidence that these are the ova of Jousseaumia. Nor was I successful, after 

 a prolonged and careful search, in finding any larval or embryonic forms much younger 

 than the specimen shown in fig. 27, though in every coral there was an abundance of 

 young forms representing every stage of later growth. The specimen depicted in 

 fig. 27 displays clearly the larval shell or prodissoconch, with its rectilinear hinge- 

 line and internal ligament. A single-growth lamina has been added at the edge of 

 the prodissoconch, so the animal cannot have passed very far beyond the larval 

 stage. The principal organs of the body are, however, well developed. The anterior 

 and posterior adductor muscles are fully formed, as is usual in young Lamellibranchs ; 

 the foot has the geniculate characters of the adult ; the retractor and protractor 

 muscles of the foot have the same relative size and importance as in the adult ; the 

 labial palps are well formed, and in the alimentary canal all the features of the adult 

 pharynx, oesophagus, stomach, csecum, liver and intestine are clearly distinguishable. 

 Only the nerve centres and the gills retain embryonic characters. In the nervous 

 system the ganglia are still larger, relatively to the whole size of the animal, than 

 they are in adult, and the connectives are relatively very thick. In the cerebro- 

 pleural ganglia the double nature of each member of the ganglion-pair, only 

 recognisable in section in the adult, is evident in a surface view (fig. 27, eg.). The 

 gills are in an early stage of development and show five fenestrations, with a 

 commencement of a sixth. The organisation is much more advanced than, for 

 instance, in the youngest Scioberetia figured by Bernard (2, plate xv., fig. 4). 

 Not only is the internal organisation well advanced, but the hinge does not show the 

 characteristic teeth of the prodissoconch, although the valves have hardly grown 

 beyond the prodissoconch stage. On the contrary, there is no trace of a provinculum, 

 the anterior cardinal teeth are clearly developed, and the large lateral teeth are 

 being formed by folds of the mantle edge just above the anterior and posterior 

 adductor muscles. In the specimen shown in fig. 28 there are four growth laminae 

 outside the prodissoconch. The organisation is somewhat more advanced than in 

 fig. 27 ; in particular the nervous system and labial palps have assumed their adult 

 proportions and the gills are larger and have acquired seven fenestrations, but as yet no 

 interfilamentar junctions. The anterior cardinal teeth of the hinge are more distinct 

 and clearly interlock with one another, and a deposition of calcareous matter round 



2 L 2 



