418 
zone for purposes such as these. It is found that these 
hard structures tend to degenerate and disappear both in 
the pelagic and deep-sea regions. 
It is a most remarkable fact that almost all these shore 
animals in their early development from the egg pass 
through free-swimming larval stages which are closely 
alike in form for very widely different zoological groups. 
As a familiar example may be taken the case of the 
common oyster. The egg of the oyster develops into a 
peculiar free-swimming larva known asa Trochosphere. 
It is globular in form and divided by a transverse band 
of cilia into a smaller anterior and larger posterior 
area. The mouth opens just behind the ciliated band. 
The larva swims actively by means of its cilia. After a 
time it develops a pair of shells, and becomes meta- 
morphosed into an oyster, and attaches itself immovably 
by one of its shells to the sea-bottom. Its shells increase 
in size and thickness, and form a protection against its 
enemies. This same trochosphere larva is common to a 
very large number of Mollusca of all varieties and shapes 
in the adult condition, and an essentially similar trocho- 
sphere is common to a large number of annelids. It is 
most remarkable that there should be so close a re- 
semblance between the larva of two adult forms so widely 
different in all respects as an oysterand a worm. An old 
explanation of such facts was that such actively-moving 
larvee were contrivances for procuring the wide diffusion 
of sedentary or less active adult forms, which might thus 
be conceived as of later origin than the forms themseives. 
But if this were the case, it is inconceivable that having 
arisen from so widely different starting points, the larvae 
should have attained so closely similar a structure. The 
only real explanation of the matter is that the common 
larval form represents a common ancestor, from which 
the various adult forms, in the existence of which it is now 
only a phase, diverged. There was thusa common freely- 
swimming ancestor of the annelids and mollusca, and it 
seems probable that the entire littoral fauna must have 
been derived originally in remote antiquity from small 
primitive and simply organised free-swimming ancestors. 
All evidence seems further to point to the conclusion that 
the primitive ancestors of all plants were also free- 
swimming. The free-swimming ancestral representatives 
of life no doubt partly inhabited the open sea, leading a 
pelagic existence, partly swarmed in sheltered bays and 
pools on the coasts, as the larve of the littoral animals do 
now. The free-swimming plants gradually produced 
attached descendants, which colonised the shores, and 
the animals, finding there a supply of food, gradually 
adapted themselves to the more complicated conditions 
of shore life. The late Prof. Balfour, in his far-famed 
work on “Embryology,” in discussing the character of 
larvee of the kind under consideration, spoke of them as 
possibly reproducing the characters of some ancestral 
forms “ which may have existed when all marine animals 
were free swimming.”! 
A peculiar instance in which there can be but little 
doubt, is that of the common barnacles. These in the 
adult condition are firmly fixed to supports of various 
kinds, and withstand the most violent action of the surf. 
The common acorn barnacles cover the most exposed 
bare rocks of our coasts, where the waves are heaviest, 
and nothing else can live. They have developed the 
Stoutest of shells to protect themselves. In the young 
larval condition, however, they are actively free-swimming 
larvee of typical crustacean structure, evidently adapted 
for pelagic existence, and to be found in swarms at the 
sea-surface, actively engaged in it. They attach them- 
selves, and become immovably adherent and sedentary, 
and invested by a shell. There can be no dovbt in this 
case that the locomotive larva represents the ancestral 
pian for allied crustacea still exist in abundance as 
adults. 
* F. M. Balfour, ‘‘ Comparative Embryology,” vol. ii. p. 305. 
NATURE 
[Sepz. 3, 1885 
A most important instance is that of the Echinoderms, 
the adults of the various groups of which, the sea-urchins 
starfish, brittle stars, holothurians, and crinoids are most, 
widely different in form, and adapted in most various 
ways to shore life. Yet these all pass through free 
swimming larval stages which are most remarkably alike. 
Supposing the adult forms to have been antecedent, it is 
quite impossible that a series of larvze could have been 
developed independently from starfish, echini, holo- 
thurians, and brittle stars, and have attained this remark- 
able coincidence of structure. This common larval form 
must represent the ancestral condition, the free-swimming 
pelagic ancestor from which the echinoderms have 
sprung. 
The fixed and inert sponges are developed from free- 
swimming ciliated larve, and Prof. W. J. Sollas! has 
observed that the young larve of the sponge Oscarel/la 
Jobularis are retained longer within the parent in the case 
of specimens occurring on the coast of Brittany than in 
that of specimens found in the Mediterranean. He 
attributes this difference to the influence of the quieter 
sea and absence of tides in the latter case. The larvee 
have come to be longer retained where the zisk of their 
loss by current and tide is greater. By the gradual 
action of similar influences, no doubt, the loss of larval 
stages in so many instances has come about. It is 
probable that there is a special tendency to such loss in 
the case of deep-sea animals. Hoek” has recorded the 
loss of the nauplius stage as a free-swimming one in the 
case of a deep-sea scapellum froma depth of 506 fathoms. 
One of the best examples of the special adaptation by 
modification of animals sprung from pelagic ancestors for 
littoral existence is that of the Madreporarian corals, the 
far-famed builders of reefs. Each coral colony is sprung 
from a locomotive planula larva, swimming by means of 
cilia. The larva attaches itself, and developes into a polyp, 
and acquires a hard skeleton, and by budding produces a 
large colonial stock. The massive stocks thus formed 
and strengthened form reefs which are barriers to the 
waves. They flourish in the water churned by themselves 
into surf, and thus specially aérated and fitted for their 
respiration, and between their branches and interstices they 
sift out the fine pelagic animals on which they feed from 
the surface water. Probably the advantages thus gained 
is the cause of their assumption of the colonial form and 
development of their stout and massive skeletons. Poss- 
ibly this is the reason why scarcely any colonial Madre- 
poraria occur in deep water, although other colonial 
animals are abundant in the depths. 
The origin of the vertebrata is a complex question, but 
they are probably sprung from a very simple free-swimming 
ancestor, as is shown by the survival of a simple ciliated 
gastrula as an early stage in the developmental history of 
Amphioxus. An exactly similar developmental stage pre- 
cedes the trochosphere form in the oyster, and the charac- 
teristic larvae in the case of the echinoderms, and occurs as 
an early stage in a wide range of other forms. From this 
ciliated gastrula develops Amphioxus, one of the most in- 
teresting components of the fauna of the coasts, one of the 
most primitive of vertebrates now existing. The Ascidians, 
which are in the adult condition as inhabitants of the 
coasts, mere inert sacs, extreme instances of degenera- 
tion, are derived from free-swimming larve of pelagic 
habits which show distinct vertebrate structure and have 
myelonic eyes, which, as Prof. Lankester has pointed out, 
could only have originated in an animal of pelagic habits. 
The Ascidians, before reaching their vertebrate larva 
stage, pass through a gastrula stage like Amphioxus. It 
is possible, therefore, that their ancestors have twice 
taken from pelagic to littoral existence, having relinquished 
the shore for a period after their first experience of it, and 
returned to it again ; whilst some of their close allies, such 
2 W. J. Sollas, Quart. Journ. Micro. Sct., 1884, p. 612. 34 
? Report on the Cirripedia. Challenger Report, Zoology, vol. vili. p. 75- 
