848 REPORT—1890. 
that these free larvee are of smaller size than the ancestral forms to which they 
correspond, 
If, on the other hand, in spite of these powerful modifying causes, we do find a 
particular larval form occurring widely and in groups not very closely akin, then 
we certainly are justified in attaching great importance to it, and in regarding it 
as having strong claims to be accepted as ancestral for these groups. 
Concerning these larval forms, and their possible ancestral significance, our 
knowledge has made no great advance since the publication of Balfour's memorable 
chapter on this subject; and I propose merely to allude briefly to a few of the 
more striking instances. 
The earliest, the most widely spread, and the most famous of larval forms is the 
gastrula, which occurs in a simple or in a modified form in some members of each of 
the large animal groups. It is generally admitted that its significance is the same in 
all cases, and the evidence is very strong in favour of regarding it as a stage ances- 
tral for all Metazoa. The difficulty arising from its varying mode of development 
in different forms is, however, still unsolved, and embryologists are not yet agreed 
whether the invaginate or delaminate form is the more primitive. In fayour of the 
former is its much wider occurrence; in favour of the latter the fact that it is easy 
to picture a series of stages leading gradually from a unicellular protozoon to a 
blastula, a diblastula, and ultimately a gastrula, each stage being a distinct advance, 
both morphological and physiological, on the preceding stage; while in the case of 
the invaginate gastrula it is not easy to imagine any advantage resulting from a 
flattening or slight pitting in of one part of the surface, sufficient to lead to its 
preservation and further development. 
Of larval forms later than the gastrula, the most important by far is the Pilidium 
larva, from which it is possible, as Balfour has shown, that the slightly later Echino- 
derm larva, as well as the widely spread Trochosphere larva, may both be derived. 
Balfour concludes that the larval forms of all Coelomata, excluding the crustacea 
and vertebrates, may be derived from one common type, which is most nearly repre- 
sented now by the Pilidium larva, and which ‘ was an organism something like a 
Medusa, with a radial symmetry.’ The tendency of recent phylogenetic specula- 
tions is to accept this in full, and to regard as the ancestor of Turbellarians and of 
all higher forms, a jelly-fish or ctenophoran, which in place of swimming freely 
has taken to crawling on the sea bottom. 
Of the two groups excluded above, the crustacea and the vertebrata, the interest of 
the former centres inthe much discussed problem of the significance of the Nauplius 
larva. There is now a fairly general agreement that the primitive crustacea were 
types akin to the phyllopods, z.e., forms with elongated and many-segmented bodies, 
and a large number of pairs of similar appendages. If this is correct, then the 
explanation of the Nauplius stage must be afforded by the phyllopods themselves, 
and it is no use looking beyond this group for it. A Nauplius larva occurs in other 
crustacea merely because they have inherited from their phyllopod ancestors the 
tendency to develop such a stage, and it is quite legitimate to hold that higher 
crustaceans are descended from phyllopods, and that the Nauplius represents in more 
or less modified form an earlier ancestor of the phyllopods themselves. 
As to the Nauplius itself the first thing to note is that though an early larval 
form it cannot bea very primitive form, for it is already an unmistakable crustacean ; 
the absence of cilia, the formation of a cuticular investment, the presence of jointed 
schizopodous limbs, together with other anatomical characters, proving this point 
conclusively. It follows therefore either that the earlier and more primitive stages 
are entirely omitted in the development of crustacea, or else that the Nauplius 
represents such an early ancestral stage with crustacean characters, which properly 
belong to a later stage, thrown back upon it and precociously developed. 
The latter explanation is the one usually adopted; but before the question can 
be finally decided more accurate observations than we at present possess are needed 
concerning the stages intermediate between the egg and the Nauplius. 
The absence of a heart in the Nauplius may reasonably be associated with the 
small size of the larva. 
Concerning the larval forms of vertebrates, it is only in Amphioxus and the 
