1. Under artificial conditions more than 10 per cent. of tho larvae of Echinus miliaris exhibited the situs inversus.

2. So far as I could examine, the internal as well as external structures of such abnormal larvae were mirror-images of those of the normal larva.

3. The young sea-urchins metamorphosed from such inverse larvae showed no abnormal features externally.

4. The manner in which such abnormal larvae departed from the normal development seems to be analogous to that in the case of ‘compensatory hypertypy’ in the claws of Alpheus.

5. In an early stage of the normally-developing happens sometimes that the left dorsal pore becomes obliterated. This seems to be associated with the shifting of the pore towards the mid-dorsal line. The hydrocoele, thus deprived of its communication with the exterior, ceases to develop and then degeneration of the whole water-vascular system sets in.

6. The right anterior coelom, on the other hand, is now evoked to realize its latent potentiality of producing a hydrocoele (homoeosis). The degenerating left hydrocoele gives place to a newly-appearing right hydrocoele.

7. The right hydrocoele stimulates its adjoining tissues to give rise together to an echinus-rudiment.

8. The external factor or factors which cause the obliteration of the dorsal pore could not be found. This probably is connected with the presence of too much diatom-food and other micro-organisms in the culture jar.

9. If a new dorsal pore is formed on the left side before the degeneration of the left hydrocoele sets in, the developing power of the latter will thereby be revived. If sufficiently fed a double-hydrocoele larva will result under such a condition.

10. If, while the left hydrocoele is arrested in its development and then degenerates, the right anterior coelom fails to develop a new hydrocoele presumably from want of sufficient food, a larva devoid of hydrocoele will result.

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