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Clark, one of my assistants, has made very good drawings of all its stages of growth, and of various other hydroid medusae peculiar to this coast. Mr. Stimpson, another very promising young naturalist, who has been connected with me for some time in the same capacity, draws the crustacea and bryozoa, of which there are also a good many new ones here.
At this time the constrictions have deepened, so that the margins of all the successive divisions of the little Hydroid are very prominent, and the whole animal looks like a pile of saucers, or of disks with scalloped edges and the convex side turned downward.
In this curious Acalephian Hydroid, or Physalia, the individuality of function is even more marked than in the Hydractinia.
The barren Platygaster larva, the fertile Cecidomyia larva, the fertile Aphis larva, the fertile Chironomus pupa, the fertile hydroid polype, and the fertile adult queen bee are simply animals in different degrees of organization, and with reproductive systems differing not in quality, but in the greater or less rapidity of their development as compared with the rest of the body.
Investigations had been made on the Hydroid stock, described as Coryne, and upon its Medusoid offspring, described as Sarsia, named after the naturalist Sars, whose beautiful papers upon this class of animals have associated his name with it; but the investigations by which all these facts have been associated in one connected series are very recent.
I subjoin a woodcut of a Silurian Coral, which does not, however, show the peculiar internal structure, but gives some idea of the general appearance of the old Hydroid Corals. We have but one Acalephian Coral now living, the Millepore; and it was by comparing that with these ancient ones that I first detected their relation to the Acalephs.
In the Hydroids, the organization does not rise above the simple digestive cavity inclosed by the double body-wall; and we might not suspect their relation to the Acalephs, did we not see the Jelly-Fish born from the Hydroid stock.
From these ovaries a new brood of little embryos is shed in due time. There are other Hydroids giving rise to Medusae buds, from which, however, the Medusae do not separate to begin a new life, but wither on the Hydroid stock, after having come to maturity and dropped their eggs. Such is the Hydractinia polyclina.
The most numerous of the ancient Radiates are the Acalephlan Corals, combining, in the Hydroid form, the Polyp-like mode of life, habits, and general appearance with the structure of Acalephs.
If a 'Bipinnaria', a 'Brachialaria', a 'Pluteus', is competent to produce the Echinoderm, which is so widely different from it; if a hydroid polype can produce the higher Medusa; if the vermiform Trematode 'nurse' can develop within itself the very unlike 'Cercaria', it will not appear impossible that the egg, or ciliated embryo, of a sponge, for once, under special conditions, might become a hydroid polype, or the embryo of a Medusa, an Echinoderm."
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