Chapter XIII: General Conclusions (2)

The Hertwigs have recently suggested in their very interesting memoir (No. 271) that the Triploblastica are to be divided into two phyla, (1) the Enterocoela, and (2) the Pseudocoela; the former group containing the Chætopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda, Echinodermata, Enteropneusta and Chordata; and the latter the Mollusca, Polyzoa, the Rotifera, and Platyelminthes.

The Enterocoela are forms in which the primitive alimentary diverticula have given origin to the body cavity, while the major part of the muscular system has originated from the epithelial walls of these diverticula, part however being in many cases also derived from the amoeboid cells, called by them mesenchyme, by the second process of mesoblastic differentiation mentioned on p. 347.

In the Pseudocoela the muscular system has become differentiated from mesenchyme cells; while the body cavity, where it exists, is merely a split in the mesenchyme.

It is impossible for me to attempt in this place to state fully, or do justice to, the original and suggestive views contained in this paper. The general conclusion I cannot however accept. The views of the Hertwigs depend to a large extent upon the supposition that it is possible to distinguish histologically muscle cells derived from epithelial cells, from those derived from mesenchyme cells. That in many cases, and strikingly so in the Chordata, the muscle cells retain clear indications of their primitive origin from epithelial cells, I freely admit; but I do not believe either that its histological character can ever be conclusive as to the non-epithelial origin of a muscle cell, or that its derivation in the embryo from an indifferent amoeboid cell is any proof that it did not, to start with, originate from an epithelial cell.

I hold, as is clear from the preceding statements, that such immense secondary modifications have taken place in the development of the mesoblast, that no such definite conclusions can be deduced from its mode of development as the Hertwigs suppose.

In support of the view that the early character of embryonic cells is no safe index as to their phylogenetic origin, I would point to the few following facts.

(1) In the Porifera and many of the Coelenterata (Eucope polystyla, Geryonia, &c.) the hypoblast (endoderm) originates from cells, which according to the Hertwigs' views ought to be classed as mesenchyme.

(2) In numerous instances muscles which have, phylogenetically, an undoubted epithelial origin, are ontogenetically derived from cells which ought to be classed as mesenchyme. The muscles of the head in all the higher Vertebrata, in which the head cavities have disappeared, are examples of this kind; the muscles of many of the Tracheata, notably the Araneina, must also be placed in the same category.

(3) The Mollusca are considered by the Hertwigs to be typical Pseudocoela. A critical examination of the early development of the mesoblast in these forms demonstrates however that with reference to the mesoblast they must be classed in the same group as the Chætopoda. The mesoblast (Vol. II. p. 227) clearly originates as two bands of cells which grow inwards from the blastopore, and in some forms (Paludina, Vol. II. fig. 107) become divided into a splanchnic and somatic layer, with a body cavity between them. All these processes are such as are, in other instances, admitted to indicate Enterocoelous affinities.

The subsequent conversion of the mesoblast elements into amoeboid cells, out of which branched muscles are formed, is in my opinion simply due to the envelopment of the soft Molluscan body within a hard shell.

In addition to these instances I may point out that the distinction between the Pseudocoela and Enterocoela utterly breaks down in the case of the Discophora, and the Hertwigs have made no serious attempt to discuss the characters of this group in the light of their theory, and that the derivation of the Echinoderm muscles from mesenchyme cells is a difficulty which is very slightly treated.

II. LARVAL FORMS: THEIR NATURE, ORIGIN AND AFFINITIES.

Preliminary considerations. In a general way two types of development may be distinguished, viz. a foetal type and a larval type. In the foetal type animals undergo the whole or nearly the whole of their development within the egg or within the body of the parent, and are hatched in a condition closely resembling the adult; and in the larval type they are born at an earlier stage of development, in a condition differing to a greater or less extent from the adult, and reach the adult state either by a series of small steps, or by a more or less considerable metamorphosis.

The satisfactory application of embryological data to morphology depends upon a knowledge of the extent to which the record of ancestral history has been preserved in development. Unless secondary changes intervened this record would be complete; it becomes therefore of the first importance to the embryologist to study the nature and extent of the secondary changes likely to occur in the foetal or the larval state.

The principles which govern the perpetuation of variations which occur in either the larval or the foetal state are the same as those for the adult condition. Variations favourable to the survival of the species are equally likely to be perpetuated, at whatever period of life they occur, prior to the loss of the reproductive powers. The possible nature and extent of the secondary changes which may have occurred in the developmental history of forms, which have either a long larval existence, or which are born in a nearly complete condition, is primarily determined by the nature of the favourable variations which can occur in each case.

Where the development is a foetal one, the favourable variations which can most easily occur are--(1) abbreviations, (2) an increase in the amount of food-yolk stored up for the use of the developing embryo. Abbreviations take place because direct development is always simpler, and therefore more advantageous; and, owing to the fact of the foetus not being required to lead an independent existence till birth, and of its being in the meantime nourished by food-yolk, or directly by the parent, there are no physiological causes to prevent the characters of any stage of the development, _which are of functional importance during a free but not during a foetal existence_, from disappearing from the developmental history. All organs of locomotion and nutrition not required by the adult will, for this reason, obviously have a tendency to disappear or to be reduced in foetal developments; and a little consideration will shew that the ancestral stages in the development of the nervous and muscular systems, organs of sense, and digestive system will be liable to drop out or be modified, _when a simplification can thereby be effected_. The circulatory and excretory systems will not be modified to the same extent, because both of them are usually functional during foetal life.

The mechanical effects of food-yolk are very considerable, and numerous instances of its influence will be found in the earlier chapters of this work[135]. It mainly affects the early stages of development, _i.e._ the form of the gastrula, &c.

[135] For numerous instances of this kind, _vide_ Chapter XI. of
Vol. III.

The favourable variations which may occur in the free larva are much less limited than those which can occur in the foetus. Secondary characters are therefore very numerous in larvæ, and there may even be larvæ with secondary characters only, as, for instance, the larvæ of Insects.

In spite of the liability of larvæ to acquire secondary characters, there is a powerful counterbalancing influence tending towards the preservation of ancestral characters, in that larvæ are necessarily compelled at all stages of their growth to retain _in a functional state_ such systems of organs, at any rate, as are essential for a free and independent existence. It thus comes about that, in spite of the many causes tending to produce secondary changes in larvæ, there is always a better chance of larvæ repeating, in an unabbreviated form, their ancestral history, than is the case with embryos, which undergo their development within the egg.

It may be further noted as a fact which favours the relative retention by larvæ of ancestral characters, that a secondary larval stage is less likely to be repeated in development than an ancestral stage, because there is always a strong tendency for the former, which is a secondarily intercalated link in the chain of development, to drop out by the occurrence of a _reversion_ to the original type of development.

The relative chances of the ancestral history being preserved in the foetus or the larva may be summed up in the following way:--There is a greater chance of the ancestral history being _lost_ in forms which develop in the egg; and of its being _masked_ in those which are hatched as larvæ.

The evidence from existing forms undoubtedly confirms the _a priori_ considerations just urged[136]. This is well shewn by a study of the development of Echinodermata, Nemertea, Mollusca, Crustacea, and Tunicata. The free larvæ of the four first groups are more similar amongst themselves than the embryos which develop directly, and since this similarity cannot be supposed to be due to the larvæ having been modified by living under precisely similar conditions, it must be due to their retaining common ancestral characters. In the case of the _Tunicata_ the free larvæ retain much more completely than the embryos certain characters such as the notochord, the cerebrospinal canal, etc., which are known to be ancestral.

[136] It has long been known that land and freshwater forms
develop without a metamorphosis much more frequently than marine
forms. This is probably to be explained by the fact that there is
not the same possibility of a land or freshwater species
extending itself over a wide area by the agency of free larvæ,
and there is, therefore, much less advantage in the existence of
such larvæ; while the fact of such larvæ being more liable to be
preyed upon than eggs, which are either concealed, or carried
about by the parent, might render a larval stage absolutely
disadvantageous.

Types of Larvæ.--Although there is no reason to suppose that all larval forms are ancestral, yet it seems reasonable to anticipate that a certain number of the known types of larvæ would retain the characters of the ancestors of the more important phyla of the animal kingdom.

Before examining in detail the claims of various larvæ to such a character, it is necessary to consider somewhat more at length the kind of variations which are most likely to occur in larval forms.

It is probable _a priori_ that there are two kinds of larvæ, which may be distinguished as primary and secondary larvæ. Primary larvæ are more or less modified ancestral forms, which have continued uninterruptedly to develop as free larvæ from the time when they constituted the adult form of the species. Secondary larvæ are those which have become introduced into the ontogeny of species, the young of which were originally hatched with all the characters of the adult; such secondary larvæ may have originated from a diminution of food-yolk in the egg and a consequently earlier commencement of a free existence, or from a simple adaptive modification in the just hatched young. Secondary larval forms may resemble the primary larval forms in cases where the ancestral characters were retained by the embryo in its development within the egg; but in other instances their characters are probably entirely adaptive.

_Causes tending to produce secondary changes in larvæ._--The modes of action of natural selection on larvæ may probably be divided more or less artificially into two classes.

1. The changes in development directly produced by the existence of
a larval stage.
2. The adaptive changes in a larva acquired in the ordinary course
of the struggle for existence.

The changes which come under the first head consist essentially in a displacement in the order of development of certain organs. There is always a tendency in development to throw back the differentiation of the embryonic cells into definite tissues to as late a date as possible. This takes place in order to enable the changes of form, which every organ undergoes, in repeating even in an abbreviated way its phylogenetic history, to be effected with the least expenditure of energy. Owing to this tendency it comes about that when an organism is hatched as a larva many of the organs are still in an undifferentiated state, although the ancestral form which this larva represents had all its organs fully differentiated. In order, however, that the larva may be enabled to exist as an independent organism, certain sets of organs, _e.g._ the muscular, nervous, and digestive systems, have to be histologically differentiated. If the period of foetal life is shortened, an earlier differentiation of certain organs is a necessary consequence; and in almost all cases the existence of a larval stage causes a displacement in order of development of organs, the complete differentiation of many organs being retarded relatively to the muscular, nervous, and digestive systems.

The possible changes under the second head appear to be unlimited. There is, so far as I see, no possible reason why an indefinite number of organs should not be developed in larvæ to protect them from their enemies, and to enable them to compete with larvæ of other species, and so on. The only limit to such development appears to be the shortness of larval life, which is not likely to be prolonged, since, _ceteris paribus_, the more quickly maturity is reached the better it is for the species.

A very superficial examination of marine larvæ shews that there are certain peculiarities common to most of them, and it is important to determine how far such peculiarities are to be regarded as adaptive. Almost all marine larvæ are provided with well-developed organs of locomotion, and transparent bodies. These two features are precisely those which it is most essential for such larvæ to have. Organs of locomotion are important, in order that larvæ may be scattered as widely as possible, and so disseminate the species; and transparency is very important in rendering larvæ invisible, and so less liable to be preyed upon by their numerous enemies[137].

[137] The phosphorescence of many larvæ is very peculiar. I
should have anticipated that phosphorescence would have rendered
them much more liable to be captured by the forms which feed upon
them; and it is difficult to see of what advantage it can be to
them.

These considerations, coupled with the fact that almost all free-swimming animals, which have not other special means of protection, are transparent, seem to shew that the transparency of larvæ at all events is adaptive; and it is probable that organs of locomotion are in many cases specially developed, and not ancestral.

Various spinous processes on the larvæ of Crustacea and Teleostei are also examples of secondarily acquired protective organs.

These general considerations are sufficient to form a basis for the discussion of the characters of the known types of larvæ.

The following table contains a list of the more important of such larval forms:

DICYEMIDÆ.--The Infusoriform larva (vol. II. fig. 62).
PORIFERA.--(_a_) The Amphiblastula larva (fig. 215), with one-half
of the body ciliated, and the other half without cilia; (_b_) an
oval uniformly ciliated larva, which may be either solid or have
the form of a vesicle.
COELENERATA.--The planula (fig. 216).
TURBELLARIA.--(_a_) The eight-lobed larva of Müller (fig. 222);
(_b_) the larva of Götte and Metschnikoff, with some Pilidium
characters.
NEMERTEA.--The Pilidium (fig. 221).
TREMATODA.--The Cercaria.
ROTIFERA.--The Trochosphere-like larvæ of Brachionus (fig. 217) and
Lacinularia.
MOLLUSCA.--The Trochosphere larva (fig. 218), and the subsequent
Veliger larva (fig. 219).
BRACHIOPODA.--The three-lobed larva, with a postoral ring of cilia
(fig. 220).
POLYZOA.--A larval form with a single ciliated ring surrounding the
mouth, and an aboral ciliated ring or disc (fig. 228).
CHÆTOPODA.--Various larval forms with many characters like those of
the molluscan Trochosphere, frequently with distinct transverse
bands of cilia. They are classified as Atrochæ, Mesotrochæ,
Telotrochæ (fig. 225 A and fig. 226), Polytrochæ, and Monotrochæ
(fig. 225 B).
GEPHYREA NUDA.--Larval forms like those of preceding groups. A
specially characteristic larva is that of Echiurus (fig. 227).
GEPHYREA TUBICOLA.--Actinotrocha (fig. 230), with a postoral
ciliated ring of arms.
MYRIAPODA.--A functionally hexapodous larval form is common to all
the Chilognatha (vol. II. fig. 174).
INSECTA.--Various secondary larval forms.
CRUSTACEA.--The Nauplius (vol. II. fig. 208) and the Zoæa (vol. II.
fig. 210).
ECHINODERMATA.--The Auricularia (fig. 223 A), the Bipinnaria (fig.
223 B), and the Pluteus (fig. 224), and the transversely-ringed
larvæ of Crinoidea (vol. II. fig. 268). The three first of which
can be reduced to a common type (fig. 231 C).
ENTEROPNEUSTA.--Tornaria (fig. 229).
UROCHORDA (TUNICATA).--The tadpole-like larva (vol. III. fig. 8).
GANOIDEI.--A larva with a disc with adhesive papillæ in front of the
mouth (vol. III. fig. 67).
ANURUS AMPHIBIA.--The tadpole (vol. III. fig. 80).

[FIG. 215. TWO FREE STAGES IN THE DEVELOPMENT OF SYCANDRA RAPHANUS.
(After Schultze.)

A. Amphiblastula stage.
B. Stage after the ciliated cells have commenced to be invaginated.

_c.s._ segmentation cavity; _ec._ granular epiblast cells; _en._
ciliated hypoblast cells.]

[FIG. 216. THREE LARVAL STAGES OF EUCOPE PLOYSTYLA. (After Kowalevsky.)

A. Blastosphere stage with hypoblast spheres becoming budded into
the central cavity.
B. Planula stage with solid hypoblast.
C. Planula stage with a gastric cavity.

_ep._ epiblast; _hy._ hypoblast; _al._ gastric cavity.]

Of the larval forms included in the above list a certain number are probably without affinities outside the group to which they belong. This is the case with the larvæ of the Myriapoda, the Crustacean larvæ, and with the larval forms of the Chordata. I shall leave these forms out of consideration.

There are, again, some larval forms which may possibly turn out hereafter to be of importance, but from which, in the present state of our knowledge, we cannot draw any conclusions. The infusoriform larva of the Dicyemidæ, and the Cercaria of the Trematodes, are such forms.

Excluding these and certain other forms, we have finally left for consideration the larvæ of the Coelenterata, the Turbellaria, the Rotifera, the Nemertea, the Mollusca, the Polyzoa, the Brachiopoda, the Chætopoda, the Gephyrea, the Echinodermata, and the Enteropneusta.

The larvæ of these forms can be divided into two groups. The one group contains the larva of the Coelenterata or Planula, the other group the larvæ of all the other forms.

The Planula (fig. 216) is characterised by its extreme simplicity. It is a two-layered organism, with a form varying from cylindrical to oval, and usually a radial symmetry. So long as it remains free it is not usually provided with a mouth, and it is as yet uncertain whether or no the absence of a mouth is to be regarded as an ancestral character. The Planula is very probably the ancestral form of the Coelenterata.

[FIG. 217. EMBRYO OF BRACHIONUS URCEOLARIS, SHORTLY BEFORE IT IS
HATCHED. (After Salensky.)

_m._ mouth; _ms._ masticatory apparatus; _me._ mesenteron; _an._
anus; _ld._ lateral gland; _ov._ ovary; _t._ tail (foot); _tr._
trochal disc; _sg._ supraoesophageal ganglion.]

The larvæ of almost all the other groups, although they may be subdivided into a series of very distinct types, yet agree in the possession of certain common characters[138]. There is a more or less dome-shaped dorsal surface, and a flattened or concave ventral surface, containing the opening of the mouth, and usually extending posteriorly to the opening of the anus, when such is present.

[138] The larva of the Brachiopoda does not possess most of the
characters mentioned below. It is probably, all the same, a
highly differentiated larval form belonging to this group.

The dorsal dome is continued in front of the mouth to form a _large præoral lobe_.

There is usually present at first an uniform covering of cilia; but in the later larval stages there are almost always formed definite bands or rings of long cilia, by which locomotion is effected. These bands are often produced into arm-like processes.

The alimentary canal has, typically, the form of a bent tube with a ventral concavity, constituted (when an anus is present) of three sections, viz. an oesophagus, a stomach, and a rectum. The oesophagus and sometimes the rectum are epiblastic in origin, while the stomach always and the rectum usually are derived from the hypoblast[139].

[139] There is some uncertainty as to the development of the
oesophagus in the Echinodermata, but recent researches appear to
indicate that it is developed from the hypoblast.

[FIG. 218. DIAGRAM OF AN EMBRYO OF PLEUROBRANCHIDIUM. (From
Lankester.)

_f._ foot; _ot._ otocyst; _m._ mouth; _v._ velum; _ng._ nerve
ganglion; _ry._ residual yolk spheres; _shs._ shell-gland; _i._
intestine.]

To the above characters may be added a glass-like transparency; and the presence of a widish space possibly filled with gelatinous tissue, and often traversed by contractile cells, between the alimentary tract and the body wall.

Considering the very profound differences which exist between many of these larvæ, it may seem that the characters just enumerated are hardly sufficient to justify my grouping them together. It is, however, to be borne in mind that my grounds for doing so depend quite as much upon the fact that they constitute a series without any great breaks in it, as upon the existence of characters common to the whole of them. It is also worth noting that most of the characters which have been enumerated as common to the whole of these larvæ are not such secondary characters as (in accordance with the considerations used above) might be expected to arise from the fact of their being subjected to nearly similar conditions of life. Their transparency is, no doubt, such a secondary character, and it is not impossible that the existence of ciliated bands may be so also; but it is quite possible that if, as I suppose, these larvæ reproduce the characters of some ancestral form, this form may have existed at a time when all marine animals were free-swimming, and that it may, therefore, have been provided with at least one ciliated band.

[FIG. 219. LARVÆ OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE.
(From Gegenbaur.)

A. and B. Earlier and later stage of Gasteropod. C. Pteropod
(Cymbulia). _v._ velum; _c._ shell; _p._ foot; _op._ operculum; _t._
tentacle.]

[FIG. 220. LARVA OF ARGIOPE. (From Gegenbaur; after Kowalevsky.)

_m._ mantle; _b._ setæ; _d._ archenteron.]

The detailed consideration of the characters of these larvæ, given below, supports this view.

This great class of larvæ may, as already stated, be divided into a series of minor subdivisions. These subdivisions are the following:

1. The Pilidium Group.--This group is characterised by the mouth being situated nearly in the centre of the ventral surface, and by the absence of an anus. It includes the Pilidium of the Nemertines (fig. 221), and the various larvæ of marine Dendrocoela (fig. 222). At the apex of the præoral lobe a thickening of epiblast may be present, from which (fig. 232) a contractile cord sometimes passes to the oesophagus.

[FIG. 221. TWO STAGES IN THE DEVELOPMENT OF PILIDIUM. (After
Metschnikoff.)

_ae._ archenteron; _oe._ oesophagus; _st._ stomach; _am._ amnion;
_pr.d._ prostomial disc; _po.d._ metastomial disc; _c.s._ cephalic
sack (lateral pit).]

2. The Echinoderm Group.--This group (figs. 223, 224 and 231 C) is characterised by the presence of a longitudinal _postoral_ band of cilia, by the absence of special sense organs in the præoral region, and by the development of the body cavity as an outgrowth of the alimentary tract. The three typical divisions of the alimentary tract are present, and there is a more or less developed præoral lobe. This group only includes the larvæ of the Echinodermata.

3. The Trochosphere Group.--This group (figs. 225, 226) is characterised by the presence of a præoral ring of long cilia, the region in front of which forms a great part of the præoral lobe. The mouth opens immediately behind the præoral ring of cilia, and there is very often a second ring of short cilia parallel to the main ring, immediately behind the mouth. The function of the ring of short cilia is nutritive, in that its cilia are employed in bringing food to the mouth; while the function of the main ring is locomotive. A perianal patch or ring of cilia is often present (fig. 225 A), and in many forms intermediate rings are developed between the præoral and perianal rings.

[FIG. 222. A. LARVA OF EURYLEPTA AURICULATA IMMEDIATELY AFTER
HATCHING. VIEWED FROM THE SIDE. (After Hallez.) _m._ mouth.

B. MÜLLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED FROM
THE VENTRAL SURFACE. (After Müller.) The ciliated band is
represented by the black line. _m._ mouth; _u.l._ upper lip.]

The præoral lobe is usually the seat of a special thickening of epiblast, which gives rise to the supraoesophageal ganglion of the adult. On this lobe optic organs are very often developed in connection with the supraoesophageal ganglion, and a contractile band frequently passes from this region to the oesophagus.

The alimentary tract is formed of the three typical divisions.

The body cavity is not developed directly as an outgrowth of the alimentary tract, though the process by which it originates is very probably secondarily modified from a pair of alimentary outgrowths.

Paired excretory organs, opening to the exterior and into the body cavity, are often present (fig. 226 _nph_).

This type of larva is found in the Rotifera (fig. 217) (in which it is preserved in the adult state), the Chætopoda (figs. 225 and 226), the Mollusca (fig. 218), the Gephyrea nuda (fig. 227), and the Polyzoa (fig. 228)[140].

[140] For a discussion as to the structure of the Polyzoon larva,
_vide_ Vol. II. p. 305.

[FIG. 223. A. THE LARVA OF A HOLOTHUROID.

B. THE LARVA OF AN ASTEROID.

_m._ mouth; _st._ stomach; _a._ anus; _l.c._ primitive longitudinal
ciliated band; _pr.c._ præoral ciliated band.]

4. Tornaria.--This larva (fig. 229) is intermediate in most of its characters between the larvæ of the Echinodermata (more especially the Bipinnaria) and the Trochosphere. It resembles Echinoderm larvæ in the possession of a longitudinal ciliated band (divided into a præoral and a postoral ring), and in the derivation of the body cavity and water-vascular vesicle from alimentary diverticula; and it resembles the Trochosphere in the presence of sense organs on the præoral lobe, in the existence of a perianal ring of cilia, and in the possession of a contractile band passing from the præoral lobe to the oesophagus.

[FIG. 224. A LARVA OF STRONGYLOCENTRUS. (From Agassiz.)

_m._ mouth; _a._ anus; _o._ oesophagus; _d._ stomach; _c._
intestine; _['v]._ and _v._ ciliated ridges; _w._ water-vascular
tube; _r._ calcareous rods.]

5. Actinotrocha.--The remarkable larva of Phoronis (fig. 230), known as Actinotrocha, is characterised by the presence of (1) a postoral and somewhat longitudinal ciliated ring produced into tentacles, and (2) a perianal ring. It is provided with a præoral lobe, and a terminal or somewhat dorsal anus.

6. The larva of the Brachiopoda articulata (fig. 220).

The relationships of the six types of larval forms thus briefly characterised have been the subject of a considerable amount of controversy, and the following suggestions on their affinities must be viewed as somewhat speculative. The Pilidium type of larva is in some important respects less highly differentiated than the larvæ of the five other groups. It is, in the first place, without an anus; and there are no grounds for supposing that the anus has become lost by retrogressive changes. If for the moment it is granted that the Pilidium larva represents more nearly than the larvæ of the other groups the ancestral type of larva, what characters are we led to assign to the ancestral form which this larva repeats?

[FIG. 225. TWO CHÆTOPOD LARVÆ. (From Gegenbaur.)

_o._ mouth; _i._ intestine; _a._ anus; _v._ præoral ciliated band;
_w._ perianal ciliated band.]

In the first place, this ancestral form, of which fig. 231 A is an ideal representation, would appear to have had a dome-shaped body, with a flattened oral surface and a rounded aboral surface. Its symmetry was radial, and in the centre of the flattened oral surface was placed the mouth, and round its edge was a ring of cilia. The passage of a Pilidium-like larva into the vermiform bilateral Platyelminth form, and therefore it may be presumed of the ancestral form which this larva repeats, is effected by the larva becoming more elongated, and by the region between the mouth and one end of the body becoming the præoral region, and by an outgrowth between the mouth and the opposite end developing into the trunk, an anus becoming placed at its extremity in the higher forms.

If what has been so far postulated is correct, it is clear that this primitive larval form bears a very close resemblance to a simplified free-swimming Coelenterate (Medusa), and that the conversion of such a radiate form into the bilateral took place, not by the elongation of the aboral surface, and the formation of an anus there, but by the unequal elongation of the oral face, an anterior part, together with the dome above it, forming a præoral lobe, and a posterior outgrowth the trunk (figs. 226 and 233); while the aboral surface became the dorsal surface.

[FIG. 226. POLYGORDIUS LARVA. (After Hatschek.)

_m._ mouth; _sg._ supraoesophageal ganglion; _nph._ nephridion;
_me.p._ mesoblastic band; _an._ anus; _ol._ stomach.]

This view fits in very well with the anatomical resemblances between the Coelenterata and the Turbellaria[141], and shews, if true, that the ventral and median position of the mouth in many Turbellaria is the primitive one.

[141] _Vide_ Vol. II. pp. 179 and 191. In this connection
attention may be called to _Coeloplana Metschnikowii_, a form
described by Kowalevsky, _Zoologischer Anzeiger_, No. 52, p. 140,
as being intermediate between the Ctenophora and the Turbellaria.
As already mentioned, there does not appear to me to be
sufficient evidence to prove that this form is not merely a
creeping Ctenophor.

[FIG. 227. LARVA OF ECHIURUS. (After Salensky.)

_m._ mouth; _an._ anus; _sg._ supraoesophageal ganglion (?).]

[FIG. 228. DIAGRAM OF A LARVA OF THE POLYZOA.

_m._ mouth; _an._ anus; _st._ stomach; _s._ ciliated disc.]

The above suggestion as to the mode of passage from the radial into the bilateral form differs largely from that usually held. Lankester[142], for instance, gives the following account of this passage:

[142] _Quart. Journ. of Micr. Science_, Vol. XVII. pp. 422-3.

"It has been recognised by various writers, but notably by Gegenbaur and Haeckel, that a condition of radiate symmetry must have preceded the condition of bilateral symmetry in animal evolution. The Diblastula may be conceived to have been at first absolutely spherical with spherical symmetry. The establishment of a mouth led necessarily to the establishment of a structural axis passing through the mouth, around which axis the body was arranged with radial symmetry. This condition is more or less perfectly maintained by many Coelenterates, and is reassumed by degradation of higher forms (Echinoderms, some Cirrhipedes, some Tunicates). The next step is the differentiation of an upper and a lower surface in relation to the horizontal position, with mouth placed anteriorly, assumed by the organism in locomotion. With the differentiation of a superior and inferior surface, a right and a left side, complementary one to the other, are necessarily also differentiated. Thus the organism becomes bilaterally symmetrical. The Coelentera are not wanting in indications of this bilateral symmetry, but for all other higher groups of animals it is a fundamental character. Probably the development of a region in front of, and dorsal to the mouth, forming the _Prostomium_, was accomplished _pari passu_ with the development of bilateral symmetry. In the radially symmetrical Coelentera we find very commonly a series of lobes of the body-wall or tentacles produced _equally_--with radial symmetry, that is to say--all round the mouth, the mouth terminating the main axis of the body--that is to say, the organism being 'telostomiate.' The later fundamental form, common to all animals above the Coelentera, is attained by shifting what was the main axis of the body--so that it may be described now as the 'enteric' axis; whilst the new main axis, that parallel with the plane of progression, passes through the dorsal region of the body running obliquely in relation to the enteric axis. Only one lobe or outgrowth of those radially disposed in the telostomiate organisms now persists. This lobe lies dorsally to the mouth, and through it runs the new main axis. This lobe is the _Prostomium_, and all the organisms which thus develop a new main axis, oblique to the old main axis, may be called prostomiate."

It will be seen from this quotation that the aboral part of the body is supposed to elongate to form the trunk, while the præoral region is derived from one of the tentacles.

Before proceeding to further considerations as to the origin of the Bilateralia, suggested by the Pilidium type of larva, it is necessary to enter into a more detailed comparison between our larval forms.

A very superficial consideration of the characters of these forms brings to light two important features in which they differ, viz.:

(1) In the presence or absence of sense organs on the præoral lobe.

[FIG. 229. TWO STAGES IN THE DEVELOPMENT OF TORNARIA. (After
Metschnikoff.)

The black lines represent the ciliated bands.

_m._ mouth; _an._ anus; _br._ branchial cleft; _ht._ heart; _c._
body cavity between splanchnic and somatic mesoblast layers; _w._
so-called water-vascular vesicle; _v._ circular blood-vessel.]

(2) In the presence or absence of outgrowths from the alimentary tract to form the body cavity.

The larvæ of the Echinodermata and Actinotrocha (?) are without sense organs on the præoral lobe, while the other types of larvæ are provided with them. Alimentary diverticula are characteristic of the larvæ of the Echinodermata and of Tornaria.

If the conclusion already arrived at to the effect that the prototype of the six larval groups was descended from a radiate ancestor is correct, it appears to follow that the nervous system, in so far as it was differentiated, had primitively a radiate form; and it is also probably true that there were alimentary diverticula in the form of radial pouches, _two_ of which may have given origin to the paired diverticula which become the body cavity in such types as the Echinodermata, Sagitta, etc. If these two points are granted, the further conclusions seem to follow--(1) that the ganglion and sense organs of the præoral lobe were secondary structures, which arose (perhaps as differentiations of an original circular nerve ring) after the assumption of a bilateral form; and (2) that the absence of these organs in the larvæ of the Echinodermata and Actinotrocha (?) implies that these larvæ retain, so far, more primitive characters than the Pilidium. The same may be said of the alimentary diverticula. There are thus indications that in two important points the Echinoderm larvæ are more primitive than the Pilidium.

[FIG. 230. ACTINOTROCHA. (After Metschnikoff.)

_m._ mouth; _an._ anus.]

The above conclusions with reference to the Pilidium and Echinoderm larvæ involve some not inconsiderable difficulties, and suggest certain points for further discussion.

In the first place it is to be noted that the above speculations render it probable that the type of nervous system from which that found in the adults of the Echinodermata, Platyelminthes, Chætopoda, Mollusca, etc., is derived, was a circumoral ring, like that of Medusæ, with which radially arranged sense organs may have been connected; and that in the Echinodermata _this form of nervous system has been retained_, while in the other types it has been modified. Its anterior part may have given rise to supraoesophageal ganglia and organs of vision; these being developed on the assumption of a bilaterally symmetrical form, and the consequent necessity arising for the sense organs to be situated at the anterior end of the body. If this view is correct, the question presents itself as to how far the posterior part of the nervous system of the Bilateralia can be regarded as derived from the primitive radiate ring.

[FIG. 231. THREE DIAGRAMS REPRESENTING THE IDEAL EVOLUTION OF
VARIOUS LARVAL FORMS.

A. Ideal ancestral larval form.
B. Larval form from which the Trochosphere larva may have been
derived.
C. Larval form from which the typical Echinoderm larva may have been
derived.

_m._ mouth; _an._ anus; _st._ stomach; _s.g._ supraoesophageal
ganglion. The black lines represent the ciliated bands.]

A circumoral nerve-ring, if longitudinally extended, might give rise to a pair of nerve-cords united _in front and behind_--exactly such a nervous system, in fact, as is present in many Nemertines[143] (the Enopla and Pelagonemertes), in Peripatus[144], and in primitive molluscan types (Chiton, Fissurella, etc.). From the lateral parts of this ring it would be easy to derive the ventral cord of the Chætopoda and Arthropoda. It is especially deserving of notice in connection with the nervous system of the above-mentioned Nemertines and Peripatus, that the commissure connecting the two nerve-cords behind is placed on the _dorsal_ side of the intestine. As is at once obvious, by referring to the diagram (fig. 231 B), this is the position this commissure ought, undoubtedly, to occupy if derived from part of a nerve-ring which originally followed more or less closely the ciliated edge of the body of the supposed radiate ancestor.

[143] _Vide_ Hubrecht, "Zur Anat. und Phys. d. Nerven-System. d.
Nemertinen," _Kön. Akad. Wiss._, Amsterdam; and "Researches on
the Nervous System of Nemertines," _Quart. Journ. of Micr.
Science_, 1880.

[144] _Vide_ F. M. Balfour, "On some points in the Anat. of
Peripatus capensis," _Quart. Journ. of Micr. Science_, Vol. XIX.
1879.

The fact of this arrangement of the nervous system being found in so primitive a type as the Nemertines tends to establish the views for which I am arguing; the absence or imperfect development of the two longitudinal cords in Turbellarians may very probably be due to the posterior part of the nerve-ring having atrophied in this group.

It is by no means certain that this arrangement of the nervous system in some Mollusca and in Peripatus is primitive, though it may be so.

In the larvæ of the Turbellaria the development of sense organs in the præoral region is very clear (fig. 222 B); but this is by no means so obvious in the case of the true Pilidium. There is in Pilidium (fig. 232 A) a thickening of epiblast at the summit of the dorsal dome, which might seem, from the analogy of Mitraria, etc. (fig. 233), to correspond to the thickening of the præoral lobe, which gives rise to the supraoesophageal ganglion; but, as a matter of fact, this part of the larva does not apparently enter into the formation of the young Nemertine (fig. 232). The peculiar metamorphosis, which takes place in the development of the Nemertine out of the Pilidium[145], may, perhaps, eventually supply an explanation of this fact; but at present it remains as a still unsolved difficulty.

[145] _Vide_ Vol. II. p. 204.

The position of the flagellum in Pilidium, and of the supraoesophageal ganglion in Mitraria, suggests a different view of the origin of the supraoesophageal ganglion from that adopted above. The position of the ganglion in Mitraria corresponds closely with that of the auditory organ in Ctenophora; and it is not impossible that the two structures may have had a common origin. If this view is correct, we must suppose that the apex of the aboral lobe has become the centre of the præoral field of the Pilidium and Trochosphere larval forms[146]--a view which fits in very well with their structure (figs. 226 and 233). The whole of the questions concerning the nervous system are still very obscure, and until further facts are brought to light no definite conclusions can be arrived at.

[146] The independent development of the supraoesophageal
ganglion and ventral nerve-cord in Chætopoda (_vide_ Kleinenberg,
_Development of Lumbricus trapezoides_) agrees very
satisfactorily with this view.

The absence of sense organs on the præoral lobe of larval Echinodermata, coupled with the structure of the nervous system of the adult, points to the conclusion that the adult Echinodermata _have retained_, and not, as is now usually held, secondarily acquired, their radial symmetry; and if this is admitted it follows that the obvious bilateral symmetry of Echinoderm larvæ is a secondary character.

[FIG. 232. A. PILIDIUM WITH AN ADVANCED NEMERTINE WORM. B. RIPE
EMBRYO OF NEMERTES IN THE POSITION IT OCCUPIES IN PILIDIUM. (Both
after Bütschli.)

_oe._ oesophagus; _st._ stomach; _i._ intestine; _pr._ proboscis;
_lp._ lateral pit (cephalic sack); _an._ amnion; _n._ nervous
system.]

The bilateral symmetry of many Coelenterate larvæ (the larva of Æginopsis, of many Acraspeda, of Actinia, &c.), coupled with the fact that a bilateral symmetry is obviously advantageous to a free-swimming form, is sufficient to shew that this supposition is by no means extravagant; while the presence of only two alimentary diverticula in Echinoderm larvæ is quite in accord with the presence of a single pair of perigastric chambers in the early larva of Actinia, though it must be admitted that the derivation of the water-vascular system from the left diverticulum is not easy to understand on this view.

A difficulty in the above speculation is presented by the fact of the anus of the Echinodermata being the permanent blastopore, and arising prior to the mouth. If this fact has any special significance, it becomes difficult to regard the larva of Echinoderms and that of the other types as in any way related; but if the views already urged, in a previous section on the germinal layers, as to the unimportance of the blastopore, are admitted, the fact of the anus coinciding with the blastopore ceases to be a difficulty. As may be seen, by referring to fig. 231 C, the anus is placed on the dorsal side of the ciliated band. This position for the anus adapts itself to the view that the Echinoderm larva had originally a radial symmetry, _with the anus placed at the aboral apex_, and that, with the elongation of the larva on the attainment of a bilateral symmetry, the aboral apex became shifted to the present position of the anus.

It may be noticed that the obscure points connected with the absence of a body cavity in most adult Platyelminthes, which have already been dealt with in the section of this chapter devoted to the germinal layers, present themselves again here; and that it is necessary to assume either that alimentary diverticula, like those in the Echinodermata, were primitively present in the Platyelminthes, but have now disappeared from the ontogeny of this group, or that the alimentary diverticula have not become separated from the alimentary tract.

So far the conclusion has been reached that the archetype of the six types of larvæ had a radiate form, and that amongst existing larvæ it is most nearly approached in general shape and in the form of the alimentary canal by the Pilidium group, and in certain other particulars by the Echinoderm larvæ.

The edge of the oral disc of the larval archetype was probably armed with a ciliated ring, from which the ciliated ring of the Pilidium type and of the Echinodermata was most likely derived. The ciliated ring of the Pilidium varies greatly in its characters, and has not always the form of a complete ring. In Pilidium proper (fig. 232 A) it is a simple ring surrounding the edge of the oral disc. In Müller's larva of Thysanozoon (fig. 222 B) it is inclined at an axis to the oral disc, and might be called præoral, but such a term cannot be properly used in the absence of an anus.

[FIG. 233. TWO STAGES IN THE DEVELOPMENT OF MITRARIA. (After
Metschnikoff.)

_m._ mouth; _an._ anus; _sg._ supraoesophageal ganglion; _br._ and
_b._ provisional bristles; _pr.b._ præoral ciliated band.]

[FIG. 234. CYPHONAUTES (LARVA OF MEMBRANIPORA). (After Hatschek.)

_m._ mouth; _a´._ anus; _f.g._ foot gland; _x._ problematical body
(probably a bud). The aboral apex is turned downwards.]

The Echinoderm ring is oblique to the axis of the body, and, owing to the fact of its passing ventrally in front of the anus, must be called postoral.

The next point to be considered is that of the affinities of the other larval types to these two types.

The most important of all the larval types is the Trochosphere, and this type is undoubtedly more closely related to the Pilidium than to the Echinoderm larva. Mitraria amongst the Chætopods (fig. 233) has, indeed, nearly the form of a Pilidium, and mainly differs from a Pilidium in the possession of an anus and of provisional bristles; the same may be said of Cyphonautes (fig. 234) amongst the Polyzoa.

The existence of these two forms appears to shew that the præoral ciliated ring of the Trochosphere may very probably be derived directly from the circumoral ciliated ring of the Pilidium; the other ciliated rings or patches of the Trochosphere having a secondary origin.

The larva of the Brachiopoda (fig. 220), in spite of its peculiar characters, is, in all probability, more closely related to the Chætopod Trochosphere than to any other larval type. The most conspicuous point of agreement between them is, however, the possession in common of provisional setæ.

Echinoderm larvæ differ from the Trochosphere, not only in the points already alluded to, but in the character of the ciliated band. The Echinoderm band is longitudinal and postoral. As just stated, there is reason to think that the præoral band of the Trochosphere and the postoral band of the Echinoderm larva are both derived from a ciliated ring surrounding the oral disc of the prototype of these larvæ (_vide_ fig. 231). In the case of the Echinodermata the anus must have been formed on the _dorsal side_ of this ring, and in the case of the Trochosphere on the _ventral side_; and so the difference in position between the two rings was brought about. Another view with reference to these rings has been put forward by Gegenbaur and Lankester, to the effect that the præoral ring of the Trochosphere is derived from the breaking up of the single band of most Echinoderm larvæ into the two bands found in Bipinnaria (_vide_ fig. 223) and the atrophy of the posterior band. There is no doubt a good deal to be said for this origin of the præoral ring, and it is strengthened by the case of Tornaria; but the view adopted above appears to me more probable.

Actinotrocha (fig. 230) undoubtedly resembles more closely Echinoderm larvæ than the Trochosphere. Its ciliated ring has Echinoderm characters, and the growth along the line of the ciliated ring of a series of arms is very similar to what takes place in many Echinoderms. It also agrees with the Echinoderm larvæ in the absence of sense organs on the præoral lobe.

Tornaria (fig. 229) cannot be definitely united either with the Trochosphere or with the Echinoderm larval type. It has important characters in common with both of these groups, and the mixture of these characters renders it a very striking and well-defined larval form.

Phylogenetic conclusions. The phylogenetic conclusions which follow from the above views remain to be dealt with. The fact that all the larvæ of the groups above the Coelenterata can be reduced to a common type seems to indicate that all the higher groups are descended from a single stem.

Considering that the larvæ of comparatively few groups have persisted, no conclusions as to affinities can be drawn from the absence of a larva in any group; and the presence in two groups of a common larval form may be taken as proving a common descent, but does not necessarily shew any close affinity.

There is every reason to believe that the types with a Trochosphere larva, viz. the Rotifera, the Mollusca, the Chætopoda, the Gephyrea, and the Polyzoa, are descended from a common ancestral form; and it is also fairly certain there was a remote ancestor common to these forms and to the Platyelminthes. A general affinity of the Brachiopoda with the Chætopoda is more than probable. All these types, together with various other types which are nearly related to them, but have not preserved an early larval form, are descended from a bilateral ancestor. The Echinodermata, on the other hand, are probably directly descended from a radial ancestor, and have more or less completely retained their radial symmetry. How far Actinotrocha[147] is related to the Echinoderm larvæ cannot be settled. Its characters may possibly be secondary, like those of the mesotrochal larvæ of Chætopods, or they may be due to its having branched off very early from the stock common to the whole of the forms above the Coelenterata. The position of Tornaria is still more obscure. It is difficult, in the face of the peculiar water-vascular vesicle with a dorsal pore, to avoid the conclusion that it has some affinities with the Echinoderm larvæ. Such affinities would seem, on the lines of speculation adopted in this section, to prove that its affinities to the Trochosphere, striking as they appear to be, are secondary and adaptive. From this conclusion, if justified, it would follow that the Echinodermata and Enteropneusta have a remote ancestor in common, but not that the two groups are in any other way related.

[147] It is quite possible that Phoronis is in no way related to
the other Gephyrea.

General conclusions and summary. Starting from the demonstrated fact that the larval forms of a number of widely separated types above the Coelenterata have certain characters in common, it has been _provisionally_ assumed that the characters have been inherited from a common ancestor; and an attempt has been made to determine (1) the characters of the prototype of all these larvæ, and (2) the mutual relations of the larval forms in question. This attempt started with certain more or less plausible suggestions, the truth of which can only be tested by the coherence of the results which follow from them, and their capacity to explain all the facts.

The results arrived at may be summarised as follows:

1. The larval forms above the Coelenterata may be divided into six groups enumerated on pages 370 to 373.

2. The prototype of all these groups was an organism something like a Medusa, with a radial symmetry. The mouth was placed in the centre of a flattened ventral surface. The aboral surface was dome-shaped. Round the edge of the oral surface was a ciliated ring, and probably a nervous ring provided with sense organs. The alimentary canal was prolonged into two or more diverticula, and there was no anus.

3. The bilaterally symmetrical types were derived from this larval form by the larva becoming oval, and the region in front of the mouth forming a præoral lobe, and that behind the mouth growing out to form the trunk. The aboral dome became the dorsal surface.

On the establishment of a bilateral symmetry the anterior part of the nervous ring gave rise (?) to the supraoesophageal ganglia, and the optic organs connected with them; while the posterior part of the nerve-ring formed (?) the ventral nerve-cords. The body cavity was developed from two of the primitive alimentary diverticula.

The usual view that radiate forms have become bilateral by the elongation of the aboral dome into the trunk is probably erroneous.

4. Pilidium is the larval form which most nearly reproduces the characters of the larval prototype in the course of its conversion into a bilateral form.

5. The Trochosphere is a completely differentiated bilateral form, in which an anus has become developed. The præoral ciliated ring of the Trochosphere is probably directly derived from the ciliated ring of Pilidium, which is itself the original ring of the prototype of all these larval forms.

6. Echinoderm larvæ, in the absence of a nerve-ganglion or special organs of sense on the præoral lobe, and in the presence of alimentary diverticula, which give rise to the body cavity, retain some characters of the prototype larva which have been lost in Pilidium. The ciliated ring of Echinoderm larvæ is probably derived directly from that of the prototype by the formation of an anus on the dorsal side of the ring. The anus was very probably originally situated at the aboral apex.

Adult Echinoderms have probably retained the radial symmetry of the forms from which they are descended, their nervous ring being directly derived from the circular nervous ring of their ancestors. They have not, as is usually supposed, secondarily acquired their radial symmetry. The bilateral symmetry of the larva is, on this view, secondary, like that of so many Coelenterate larvæ.

7. The points of similarity between Tornaria and (1) the Trochosphere and (2) the Echinoderm larvæ are probably adaptive in the one case or the other; and, while there is no difficulty in believing that those to the Trochosphere are adaptive, the presence of a water-vascular vesicle with a dorsal pore renders probable a real affinity with Echinoderm larvæ.

8. It is not possible in the present state of our knowledge to decide how far the resemblances between Actinotrocha and Echinoderm larvæ are adaptive or primary.

BIBLIOGRAPHY.

(257) Allen Thomson. _British Association Address_, 1877.

(258) A. Agassiz. "Embryology of the Ctenophoræ." _Mem. Amer. Acad. of Arts and Sciences_, Vol. X. 1874.

(259) K. E. von Baer. _Ueb. Entwicklungsgeschichte d. Thiere._ Königsberg, 1828-1837.

(260) F. M. Balfour. "A Comparison of the Early Stages in the Development of Vertebrates." _Quart. Journ. of Micr. Sci._, Vol. XV. 1875.

(261) C. Claus. _Die Typenlehre u. E. Haeckel's sg. Gastræa-theorie._ Wien, 1874.

(262) C. Claus. _Grundzüge d. Zoologie._ Marburg und Leipzig, 1879.

(263) A. Dohrn. _Der Ursprung d. Wirbelthiere u. d. Princip des Functionswechsels._ Leipzig, 1875.

(264) C. Gegenbaur. _Grundriss d. vergleichenden Anatomie._ Leipzig, 1878. _Vide_ also Translation. _Elements of Comparative Anatomy._ Macmillan & Co., 1878.

(265) A. Götte. _Entwicklungsgeschichte d. Unke._ Leipzig, 1874.

(266) E. Haeckel. _Studien z. Gastræa-theorie_, Jena, 1897; and also _Jenaische Zeitschrift_, Vols. VIII. and IX. 1874-5.