result, is the indication of a bilateral symmetry in some of the orders. In the Holothurians, for instance, there is a decided tendency toward the establishment of a posterior and anterior extremity, of a right and left, an upper and lower side of the body. In the Sea-urchins, in many of which the mouth is out of centre, placed nearer one side than the other, this tendency is still apparent, while in the three lower groups, the Star-fishes, Ophiurans, and Crinoids, it is almost entirely lost, in the equal division of identical parts radiating from a common centre. A comparison of the embryonic and adult forms in these orders, confirms entirely this classification based upon structural features. The Star-fishes, in their earlier stages, resemble the mature Ophiurans, while the Crinoids, the lowest group of all, retain throughout their whole existence many features characteristic of the embryonic conditions of the higher Echinoderms. In this principle of classification, already so fertile in results, we may hope to find, in some instances, the solution of many perplexing points respecting the structural rank of animals, the confirmation of classifications already established; in others, an insight into the true relations of groups which have hitherto been divided upon purely arbitrary grounds. * * * * * DISTRIBUTION OF LIFE IN THE OCEAN. We have seen that while our bay is rich in certain species, it is wholly deficient or but scantily supplied with others, and that the character of the animals inhabiting its waters is more or less directly connected with general physical conditions. Such an area, limited though it be, gives us some insight into the laws which, in their wider application, control the distribution of marine life along the shores of the most extensive continents. The coast of Massachusetts, taken as a whole, is like that of New England generally, a rocky coast; yet it has its sandy and muddy beaches, and though it lies for a great part open to the sea, it has nevertheless its sheltered harbors, its quiet bays and snug recesses.