50 



SPOLIA ZEYLANfCA. 



gradually disappearss. But it also seems that a microgen can grow 

 independently and become a macrogen, with or without the formation 

 of one or more endogens. Thus, side by side, we find microgen^' 

 containing offspring, and barren macrogens ; or a larva larger than 

 the average may bear a single endogen as in text figure 2. I have 

 also noted a mobile macrogen 0*5 mm. in length with only one 

 endogen. But the greatest disproj)ortion between the sizes of the 

 parent and that of the offspring which 1 have seen is represented in 

 text figures 3 and 4 ; in the former the contracted body with everted 

 rostellum measured 0'9 mm. in length, the latter was over a milh- 

 metre long. In each case the body Avas crowded mth the usual 

 large concretions, but there was only a solitary endogen. 



Fig. 5. — Encysted larva from liver of " pakku maddi " {Venus sp.). 

 Sambore river. Zeiss. 3 C. cam. luc. 



1 should mention that I have seen a tetragen, i.e. , an encysted larva 

 containing four endogens. Fig. 11 is a sketch of what I take to be 

 an endogen having emerged from a ruptured cyst, about to give rise 

 to a new cyst ; it will be noticed that it contains both large and small 

 granules ; its greatest diameter is 0-1 mm. 



The greatest number of endogens which I have counted in a single 

 oyst is upwards of twenty (Sambore river, 12-VI.-07). The example 

 shown in fig. 1 on the plate has fourteen endogens (Lake Tam- 

 blegam, Station IV., No. 11). 



I have also found the same kind of larval forms in other bivalve 

 molluscs which live in the sand and mud of the Sambore river, 

 namely, in the " irattamaddi " {Area rhombea) and in two species 

 of Venus called pakku maddi" and " valukkal maddi" (see text 

 fig. 5). In a liver preparation from the last-named I observed a 

 free larva moving swiftly along by the alternate protrusi-on and 

 retraction of its rostellum ; on the addition of sea water it pressed 

 itself between the lobules of the liver and became motionless. 



