This powerful suctorial apparatus is inn_The Origin of Vertebrates

nerve, so that here in its muscular arrangements any original segmental

arrangement of the muscles of mastication might be expected to be visible.

It consists of a large rod or piston, to which are attached powerful

longitudinal muscles; a large muscle, the basilar muscle, which assists the

piston in producing a vacuum, and annular muscles around the circular lip.

Turn now to the full-grown larval form, Ammocoetes, an animal in the case

of _Petromyzon Planeri_ as large as the full-grown Petromyzon, and seek for

this musculature. There is, apparently, no sign of it, no suctorial

apparatus whatever, only, as already mentioned, an oral chamber bounded by

the lower and upper lips and the remains of the septum between it and the

respiratory chamber--the velar folds. Attached to its walls a number of

tentacles are situated, which form a fringe around and within the mouth.

Most extraordinary is the contrast here between the larval and the adult

stages; in the former, no sign of the suctorial apparatus, but simply

tentacles and velar folds; in the latter, no sign of tentacles or of velar

folds, but a massive suctorial apparatus.

In order, then, to understand the origin of the muscles of mastication, it

is necessary to study the changes which occur at transformation, and thus

to find out how the suctorial apparatus of the adult arises. This most

important investigation has been undertaken by Miss Alcock, and owing to

the kindness of Mr. Millington, of Thetford, we have been able to obtain a

better series in the transformation process than has ever been obtained

before. Miss Alcock has not yet published her researches, but has allowed

me to make use of some of her facts.

An enormous proliferation of muscular tissue takes place with great

rapidity during this transformation, which causes the disappearance of the

tentacles, and gives origin to the suctorial apparatus. The starting point

of this proliferation can be traced back in all cases to little groups of

embryonic tissue found below the epithelial lining of the oral chamber in

Ammocoetes. Of these groups the most conspicuous one is situated at the

base of the large median ventral tentacles. Others are situated at the base

of the tentacular ridge. Further, although this extraordinary change takes

place in the {288}peripheral organ, no marked difference occurs in the

arrangement of the nerves issuing from the trigeminal motor centre, no new

nerves are formed to supply the new muscles, but every motor nerve-fibre

and the motor cell from which it arises increases enormously in size, and

these giant nerve-fibres thus formed split into innumerable filaments

corresponding with the proliferation of the muscular elements.

The clue, then, to the origin of the suctorial apparatus and of the nature

of the original organs supplied by the trigeminal is afforded in this case,

as in all other similar inquiries, by the central nervous system and its

outgoing nerves. Here is always the citadel, the fixed seat of government,

here is 'headquarters,' from which the answers to all our inquiries must

originate.

[Illustration: FIG. 114.--DISTRIBUTION OF TRIGEMINAL NERVE IN AMMOCOETES.

_ps. br._, pseudo-branchial groove; _met._, nerve to lower lip, or

metastomal nerve; _t._, nerve to tongue; _tent._, nerve to tentacles. The

mandibular and internal maxillary nerves are coloured red; the purely

sensory nerves to the external surface are coloured black.]

THE TRIGEMINAL NERVE OF AMMOCOETES.

Striking is the answer. In Fig. 114, Miss Alcock has drawn the distribution

of the trigeminal nerve as traced by her through a series of sections. It

arises, as is well known, from two separate ganglia, of which the foremost

gives rise to a purely cutaneous nerve, the ophthalmic nerve, and the

hindmost to three nerves, the most posterior of which is purely cutaneous

and passes tailwards over the ventral branchial region, as shown in the

figure; the other two nerves, both {289}of which contain motor fibres, are

called by Hatschek the mandibular and maxillary nerves. Of these the

mandibular or velar nerve (_met._) is a large, conspicuous nerve, which

arises so separately from the rest of the trigeminal as almost to deserve

the title of a separate nerve. When it leaves the large posterior ganglion,

it passes into the anterior part of the velum, runs along with the tubular

muscles, which it supplies, to the ventral surface as far as the junction

of the lower lip with the thyroid plate, and has not been followed further

by Hatschek. Miss Alcock, however, by means of serial sections, has traced

it further, and shown that at this point it turns abruptly headwards to

terminate in the muscles of the lower lip. If, then, as suggested, the

lower lip represents the metastoma--the last pair of prosomatic

appendages--then this mandibular or velar nerve represents that segmental

nerve.

The other nerve--the maxillary nerve of Hatschek--which constitutes the

larger part of the trigeminal, passes forwards from the ganglion, and at a

point somewhere about the anterior region of the eyeball, divides into two,

an external (_black_ in Fig. 114) and an internal (_red_ in Fig. 114)

nerve. The external branch is apparently entirely sensory, and supplies the

external surfaces of the upper and lower lips. The internal branch is

mainly motor, and supplies the muscles of the upper lip; it contains also

the nerves of the tentacles.

The nerve to the median ventral tentacle (_t._) or tongue leaves the

internal division of the maxillary immediately after its separation from

the external; it runs ventralwards, and at the same time passes internally

until it reaches a position between the muco-cartilage and the epithelium

lining the cavity of the throat. It then turns, and passing posteriorly

(towards the tail) to the point where the median ventral tentacle is

attached to the lower lip, it supplies some very rudimentary-looking

muscles which run from the tentacle to the adjoining surface, and no doubt

serve to move the tentacle from side to side. A portion of the nerve still

continues to run along the side of the median ventral ridge, as far back as

the point where the muscles of the hyoid segment pass round to the ventral

side between the velum and the thyroid; in fact, this small nerve passes

along the whole length of the median ventral ridge.

This description shows that the trigeminal nerve divides itself into two

groups: the one represented black in the figure, which is purely cutaneous

and sensory, corresponding, in the main, according {290}to my theory, to

the epimeral nerves of Limulus; the other coloured red, which supplies

muscles belonging to the visceral or splanchnic muscle-group, and contains

also the nerves to the tentacles.

This latter group, which is formed by two distinct well-defined nerves,

viz. the mandibular and the internal branch of the maxillary, corresponds,

according to my theory, to the amalgamated nerves of the prosomatic

appendages, and is clearly divisible into three distinct nerves--

1. The lower lip-nerve or the metastomal nerve (_met._).

2. The tongue-nerve (_t._).

3. The nerve (_tent._) to the upper lip and tentacles.

Of these three pairs of nerves it is suggested that the first pair were

derived from the nerves to the metastomal appendage. The second pair of

nerves ought, on this theory, originally to have supplied the pair of

appendages immediately in front of the metastoma--that is, the pair of

ectognaths, and therefore the ventral pair of tentacles, known as the

tongue, would represent the last remnant of these ectognaths. Similarly,

the other tentacles would represent the endognaths, and therefore the third

pair of nerves would represent the fused nerves to these concentrated

endognaths, which, in the Eurypterids, stand aloof from the ectognaths.

Let us consider these three propositions separately. In the first place,

have we any right to attribute segmental value to the mandibular nerve?

What evidence is there of segments in this region in Ammocoetes?

THE SEGMENT OF THE LOWER LIP, OR METASTOMAL SEGMENT.

We have seen that in the branchial or mesosomatic region the segments

corresponding to the mesosomatic appendages were mapped out by means of

their supporting or skeletal structures, their segmental muscles, and their

nervous arrangements, as well as by the arrangement of the branchiæ.

Similarly, the segments in front of the branchial region, corresponding to

the prosomatic appendages, ought to be definable by the same means,

although, owing to the absence of branchiæ and the greater concentration in

this region, the separate segments would probably not be so conspicuous.

The last segment considered was the segment belonging to the VIIth nerve

corresponding to the opercular appendages of the {291}Eurypterid. The

segment immediately in front of this is the next for consideration, viz.

that corresponding to the chilarial appendages or metastoma; and as the

basal part of this pair of appendages was fused with the basal part of the

operculum, the one cannot be discussed without the other; therefore, the

segment to which the lower lip belongs must be considered in connection

with and not apart from the thyro-hyoid segments already dealt with.

In Chapter V., p. 188, I stated that the supporting bars of the foremost

mesosomatic segments, the thyro-hyoid segments, differed from the

cartilaginous bars of the branchial segments, in that they were composed of

muco-cartilage. Also in addition to the muco-cartilaginous skeletal bars, a

ventral plate of muco-cartilage exists in Ammocoetes which covers over the

thyroid gland.

Similarly in the prosomatic segments the skeletal bars are composed of

muco-cartilage and the ventral plate of muco-cartilage continues forward as

the plate of the lower lip. It is of special interest, in connection with

the segments indicated by such supporting structures, to find that this

special tissue is entirely confined to the head-region, and disappears

absolutely at transformation, thus indicating the ancestral nature of the

segments marked out by its presence.

This muco-cartilaginous skeleton is the key to the whole position, and

requires, therefore, to be understood. It is of great importance, not only

because it demonstrates the position of the segments in Ammocoetes which

characterized its invertebrate ancestor, but also because it possesses a

structure remarkably similar to that found in the head-plates of the most

ancient fishes. For the present I will confine myself to the consideration

of this muco-cartilaginous skeleton as evidence of the relationship of

Ammocoetes to the Eurypterids, and in the next chapter will show how

absolutely the same skeleton corresponds to that of the Cephalaspidæ, so

that Ammocoetes is really a slightly modified Cephalaspid, the larval form

of which was Eurypterid in character.

{292}[Illustration: FIG. 115.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.

_Tr._, trabeculæ; _Pit._, pituitary space; _Inf._, infundibulum; _Ser._,

median serrated flange of velar folds.] {293}[Illustration: FIG.

116.--HORIZONTAL SECTION THROUGH THE ANTERIOR PART OF AMMOCOETES,

IMMEDIATELY VENTRALLY TO THE AUDITORY CAPSULE.

_sk_1_-_sk_5_, skeletal bars; _m_1_-_m_5_, striated visceral muscles;

_mt_1_-_mt_4_, tubular muscles; _br_1_-_br_3_, branchiæ; _tr._, trabeculæ;

_inf._, infundibulum; _ped._, pedicle; _V._, trigeminal nerve.

Muco-cartilage, _red_; soft cartilage, blue; hard cartilage, purple.]

{294}[Illustration: FIG. 117.--SAGITTAL LATERAL SECTION THROUGH THE

ANTERIOR PART OF AMMOCOETES.

Lettering and colouring same as in Fig. 116. _aud._, auditory capsule;

_j.v._, jugular vein.]

In Chapter IV., Figs. 63, 64, I have given a representation of the ventral

and dorsal views of an Ammocoetes cut in half horizontally. Such a section

shows with great clearness the series of branchial appendages with their

segmental muscles and cartilaginous bars which form the branchial segments

innervated by the IXth and Xth nerves, according to my view of the

branchial unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid

or opercular appendage, which is clearly serially homologous with the other

branchial bars, is composed of muco-cartilage, and not of cartilage. If we

follow this series of horizontal sections nearer to the origin of the

cartilaginous bars from the sub-chordal cartilaginous rod on each side of

the notochord, we obtain a picture, as in Fig. 116, in which each branchial

segment is defined by the section of the branchial cartilaginous bar

(_sk_4_, _sk_5_), by the section of the separate branchiæ (_br_2_, _br_3_),

and by the separate segmental muscles arranged round each bar, these

muscles being partly ordinary striated (_m_4_, _m_5_), partly tubular

(_mt_3_, _mt_4_). The uppermost of these branchial segments shows the same

arrangement; (_sk_3_) is the branchial skeletal bar, which is now composed

of muco-cartilage, not cartilage; (_br_1_) is the branchiæ in the same

situation as the others, but here composed of glandular rather than of

respiratory epithelium, while the ordinary striated branchial muscles of

this segment are marked as (_m_3_), being separated from the tubular

muscles of the segment (_mt_2_), owing to the large size of the blood-space

in which these latter muscles are lying. In front of this segment so

defined we see again another well-marked skeletal bar (_sk_2_) of

muco-cartilage, evidently indicating a similar segment anterior to the

hyoid segment. In connection with this bar there are no branchiæ, but again

we see two sets of visceral muscles, the one ordinary striated, marked

(_m_2_), and the other tubular, marked (_mt_1_). Here, then, the section

indicates the existence of a segment of the same character as the

posteriorly situated branchial segments but belonging to a non-branchial

region--a segment which would represent a non-branchial appendage, the

last, therefore, of the prosomatic appendages. Let us, then, follow

{295}out these two segmental muco-cartilaginous bars and their attendant

muscles, and see to what sort of segments their investigation leads.

The bar which comes first for consideration (_sk_3_) arises immediately

behind the auditory capsule from the first branchial cartilage very soon

after it leaves the sub-chordal cartilaginous ligament; the soft cartilage

of the sub-chordal ligament ceases abruptly in its extension along the

notochord at the place where the hard cartilage of the parachordal joins

it, and in a sense it may be said to leave the notochord at this place and

pass into the basal part of the first branchial bar. The most anterior

continuation of this branchial system is this muco-cartilaginous bar

(_sk_3_), which passes forward and ventralwards, being separated from the

axial line by the auditory capsule (_cf._ Fig. 118, A, B, C). Its position

is well seen in a sagittal section, such as Fig. 117. It follows absolutely

the line of the pseudo-branchial groove (_ps. br._, Fig. 114), and

ventrally joins the plate of muco-cartilage which covers the thyroid gland.

It forms a thickened border to this plate anteriorly, just as the branchial

cartilaginous bars border it posteriorly. In fact, it behaves with respect

to the hyoid segment in a manner similar to the rest of the cartilaginous

bars with respect to their respective segments.

It represents, although composed of muco-cartilage, the cartilaginous bar

of the operculum in Limulus, which also forms the termination of the

branchial cartilaginous system, as fully explained in Chapter III.; it may

therefore be called the opercular bar.

The next bar (_sk_2_) is extremely interesting, as we are now out of the

branchial or mesosomatic region, and into the region corresponding to the

prosoma. It starts from a cartilaginous projection made of hard cartilage,

just in front of the auditory capsule, called by Parker the 'pedicle of the

pterygoid'--a projection (_ped._) which defines the posterior limit of the

trabeculæ on each side, where they join on to the parachordals,--and

winding round and below the auditory capsule, joins the opercular bar

(_cf._ Fig. 118), to pass thence into and form part of the

muco-cartilaginous plate of the lower lip. In the section figured (Fig.

116), this projection of hard cartilage is not directly continuous with

(_sk_2_), owing to a slight curvature in the bar; the next few sections

show clearly the connection between (_ped._) and (_sk_2_), and consequently

the complete separation by means of this bar of the hyoid segment from the

segment in front.

{296}[Illustration: FIG. 118.--SKELETON OF HEAD-REGION OF AMMOCOETES. A,

LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW.

Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_.

_sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na.

cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._,

notochord.]

{297}In the figures, the hard cartilage is coloured purple, the soft

cartilage blue, and the muco-cartilage red, so that the position of this

bar is well shown. This bar may be looked upon as bearing the same relation

to the muco-cartilaginous plate of the lower lip as the opercular bar does

to the muco-cartilaginous plate over the thyroid; and seeing that these two

plates form one continuous ventral head-shield of muco-cartilage (Fig. 118,

B), and also that this bar fuses with the opercular bar, we may conclude

that the segment represented by the lower lip is closely connected with the

hyoid or opercular segments. In other words, if the lower lip arose from

the metastoma, then this pair of skeletal bars might be called the

metastomal bars, which formed the supporting skeleton of the last pair of

prosomatic appendages and, as is likely enough, arose in connection with

the posterior lateral horns of the plastron; these posterior lateral horns,

like the rest of the plastron, would give rise to hard cartilage, and so

form in Ammocoetes the two lateral so-called pterygoid projections.

In the branchial region the muscles which marked out each branchial segment

were of two kinds--ordinary striated visceral muscles and tubular muscles.

Of these the former represented the dorso-ventral muscles of the branchial

appendages, while the latter formed a separate group of dorso-ventral

muscles with a separate innervation which may have been originally the

segmental veno-pericardial muscles so characteristic of Limulus and the

scorpions. In Figs. 116, 117, the grouping of these muscles in each

branchial segment is well shown, and it is immediately seen that the hyoid

segment possesses its group of striated visceral muscles (_m_3_) supplied

by the VIIth nerve in the same manner as the posterior groups, as has

already been pointed out by Miss Alcock in her previous paper. Passing to

the segment in front, Fig. 116 shows that the group of visceral muscles

(_m_2_) corresponds in relative position with respect to the metastomal bar

to the hyoid muscles with respect to the opercular bar or to the branchial

visceral muscles with respect to each branchial bar. What, then, is this

muscular group? The series of sections show that these are the

dorso-ventral muscles belonging to the lower lip, which, as seen in Fig.

119 (_M._), form a well-marked muscular sheet, whose fibres interlace

across the mid-ventral line of the lower lip. This group of lower

lip-muscles is very suggestive, for these muscles arise, not from the

trabeculæ, but from the front dorsal region of the cranium, just in front

of the two lateral {298}eyes. In Fig. 117 the dorsal part is seen cut

across on its way to its dorsal attachment. Such an origin is reminiscent

of the tergo-coxal group of muscles, arising, as they do, from the

primordial cranium and the tergal carapace, and suggests at once that when

the chilarial appendages expanded to form a metastoma, their tergo-coxal

muscles formed a sheet of muscles similar to those of the lower lip of

Ammocoetes, by which the movements of the metastoma were effected. The

posterior limit of these muscles ventrally marks out the junction of the

segment of the lower lip with that of the thyroid; in other words,

indicates where the metastoma had fused ventrally with the operculum (Fig.

117).

[Illustration: FIG. 119.--VENTRAL VIEW OF HEAD-REGION OF AMMOCOETES.

_Th._, thyroid gland; _M._, lower lip, with its muscles.]

Besides the striated visceral muscles, each branchial segment possesses its

own tubular muscles, shown in Fig. 116 (_mt_3_) and (_mt_4_). As the

section shows, there is clearly a group of tubular muscle-fibres belonging

to the hyoid segment (_mt_2_), and also another group belonging to the

segment in front of the hyoid (_mt_1_); so that, judging from this section,

each of these segments possesses its own tubular musculature just as do the

branchial segments, the difference being that the tubular muscles are more

separated from the striated visceral group than in the true branchial

segments, owing to the size of the blood-spaces surrounding them. What,

then, are these two groups of muscles? Tracing them in the series of

sections, both groups are seen to belong to the system of velar muscles,

forming an anterior and a posterior group respectively; and we see,

further, that there is not the slightest trace of any tubular muscles

anterior to these muscles of the velum.

In the living Ammocoetes the velar folds on each side can be seen {299}to

move synchronously with the movements of respiration, contracting at each

expiration, and thus closing the slit by which the oral and respiratory

chambers communicate, and so forcing the waters of respiration through the

gill-slits, as described by Schneider. Such a fact is clear evidence that

these tubular muscles of the velar folds belong to the same series as the

tubular muscles of the branchial segments, so that if, as I have already

suggested, the latter muscles were originally the veno-pericardial muscles

of segments corresponding to the branchial appendages, then the former

would represent the veno-pericardial muscles of the segments corresponding

to the opercular and metastomal appendages. What, then, are these velar

folds, and how is it that the tubular muscles of these two segments become

the velar muscles? I will consider, in the first instance, the posterior

group of muscles (_mt_2_) in Fig. 116.

It has already been pointed out that the tubular muscles of the branchial

segments are dorso-ventral, but do not run with the ordinary constrictors,

having separate attachments and running part of their course internally to

and part externally to the ordinary constrictors. At first sight, as is

usually stated, the hyoid segment does not appear to possess tubular

muscles at all. If, however, we follow the posterior group of velar muscles

(_mt_2_), we see (Fig. 117) that they pass between the auditory capsule and

the opercular bar (_sk_3_) of muco-cartilage to reach the region of the

jugular vein (_j.v._) posteriorly to the auditory capsule, so that their

dorsal origin bears the same relation to the hyoid segment as the dorsal

attachment of the rest of the tubular muscles to their respective segments.

Further, these muscles run along the length of the velar fold, and are

attached ventrally on each side of the thyroid gland, so that their ventral

attachment also corresponds in position, as regards the hyoid segment, with

the ventral attachment of the rest of the tubular muscles as regards their

respective segments.

This ventral attachment is shown in Fig. 119 on each side of the thyroid,

and in Fig. 120 (_mt_2_); while in Fig. 117 the fibres are seen converging

to this ventral position. In other words, this large posterior muscle of

the velar folds is a dorso-ventral muscle, and would actually take the same

position in the hyoid segment as the dorso-ventral tubular muscles in the

other branchial segments, if the velum were put back into its original

position as the septum terminating the branchial chamber. Conversely, the

presence of these {300}hyoid tubular muscles in the velum gives evidence

that the opercular segment takes part in the formation of the septum, as

already suggested.

Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid

segment, which are attached to the muco-cartilage. Schaffer also speaks of

certain tubular muscles belonging to the velar group as piercing the

muco-cartilage (_h. r. s._) in his figures 24 and 25, _i.e._ the metastomal

bar, near its junction with the opercular bar. In my specimens there is a

distinct group of tubular muscles which pierce the opercular bar of

muco-cartilage at its junction with the metastomal bar, and pass into the

posterior group of velar muscles. They clearly belong to the hyoid segment,

as Miss Alcock supposed, but are not attached to the muco-cartilage. It is

possible that they represent a different group to those already considered,

and suggest the possibility that this opercular or thyro-hyoid segment is

double with respect to its original veno-pericardial muscles as well as in

other respects.

The anterior group of tubular muscles (_mt_1_, Figs. 116, 117) belonging to

the same segment as the metastomal bar must now be taken into

consideration. Very different is their origin to that of the posterior

group: they arise close up against the eye, and have given rise to

Kupffer's and Hatschek's misconception that the superior oblique muscle of

the eye arises from a part of the velar musculature. Naturally, as Neal has

pointed out, they have nothing to do with the eye-muscles; the superior

oblique muscle is plainly in its true place entirely apart from these velar

muscles, which form the foremost group of the segmental tubular muscles.

They pass into the anterior part of the velar folds and run round to the

ventral side just in the same way as does the posterior group. This

anterior group of tubular muscles represents the veno-pericardial muscle of

the segment immediately in front of the opercular, _i.e._ the metastomal

segment, and is the foremost of these veno-pericardial muscles. Its

presence shows that the velar folds, formed as they were by the breaking

down of the septum, are in reality part of two segments, viz. the opercular

and the metastomal, which have fused together in their basal parts, and by

such fusion have caused the inter-relationship between the VIIth and Vth

nerves, so apparent in the anatomy of the vertebrate cranial nerves.

A further piece of evidence that this anterior portion of the velum

{301}belongs to the same segment as the lower lip is the fact that in

addition to the tubular muscles a single ordinary striated muscle is found

in the velum which, like the muscles of the lower lip, is innervated by

this same mandibular nerve.

This muscle is attached laterally to the muco-cartilage of the metastomal

bar (_sk_2_) at its junction with the muco-cartilage of the lower lip, and

spreads out into a number of strands which are attached at intervals along

the whole length of the free anterior edge of the velum. It is the only

non-tubular muscle belonging to the velum, and by its contraction it draws

the anterior portions of the velar folds apart from each other, and so

opens the slit between them, through which the food and mud must pass.

Clearly from its position it does not belong to the original tergo-coxal

group of muscles as do those of the lower lip; it must have been one of the

intrinsic muscles of the metastoma itself.

This anterior portion of the velar folds affords yet another striking hint

of the correctness of my comparison of the lower lip segment of Ammocoetes

with the chilaria of Limulus or the metastoma of Eurypterus; for the most

dorsal anterior portion, which at its attachment possesses a wedge of

muco-cartilage, forms a separate, well-defined, rounded basal projection

marked _Ser._ in Fig. 115, and _B_ in the accompanying Fig. 120. This is

that part of the velar folds which comes together in the middle line and

closes the entrance into the respiratory chamber. The epithelial surface

here is most striking and suggestive, for it is markedly serrated, being

covered with a large number of closely-set projections or serræ. The

serration of the surface here is of so marked a character that Langerhans

considered this part of the velar folds to act as a masticating organ,

grinding and rasping the food and mud which passed through the narrow slit.

In fact, Langerhans supposed that this portion of the velum acted in a

manner closely resembling the action of the gnatho-bases of the prosomatic

appendages in Limulus or the Eurypteridæ.

This suggestion of Langerhans is surely most significant, considering that

this somewhat separate portion of the velum, to which he assigns such a

function, is in the very place where the gnathite portion of the metastomal

appendages would have been situated if it were true that the lower lip and

anterior portion of the velum of Ammocoetes were derived from the

metastoma.

In addition to this marked serrated edge the whole surface of {302}the

anterior portion of the velum is covered over with a scale-like or

tubercular pattern remarkably like the surface-ornamentation seen in many

of the members of the ancient group Eurypteridæ. In Fig. 121 I give a

picture of this surface-marking of the velum. It is striking to see that

just as in the case of the invertebrate this marking and these serræ are

formed simply by the cuticular surface of the epithelial cells; a surface

which, according to Wolff, possibly contains chitin. The interpretation

which I would give of the velar folds is therefore as follows:--

They represent the fused basal parts of the opercular and metastomal

appendages, the gnatho-bases of the latter still retaining in a reduced

degree their rasping surfaces, because, owing to their position on each

side of the opening into the respiratory chamber they were still able to

manipulate the food as it passed by them after the closure of the old

mouth.

[Illustration: FIG. 120.--AMMOCOETES CUT OPEN IN MID-VENTRAL LINE TO SHOW

POSITION OF VELUM; VELAR FOLDS REMOVED ON ONE SIDE.

_tr._, trabeculæ; vel., velum; _B._, anterior gnathic portion of velum;

_ps. br._, pseudo-branchial groove; _m_2_, muscles of lower lip segment;

_m_3_, muscles of thyro-hyoid segment; _mt_2_, insertion of tubular muscles

of velum near thyroid.]

[Illustration: FIG. 121.--SURFACE VIEW OF ANTERIOR SURFACE OF VELUM.]

The whole evidence points irresistibly to the conclusion that the

mandibular or velar nerve of the trigeminal does supply a splanchnic

{303}segment which is, in all respects, comparable with the segments

supplied by the facial, glossopharyngeal, and vagus nerves, except that it

does not possess branchiæ. This simply means that the appendages which

these nerves originally supplied were prosomatic, not mesosomatic, and

corresponded, therefore, to the chilarial or metastomal appendages.

A comparison of the ventral surface of Slimonia, as given in Fig. 8, p. 27,

with that of Ammocoetes (Fig. 119), when the thyroid gland and lower lip

muscles have been exposed to view, enables the reader to recognize at a

glance the correctness of this conclusion.

THE TENTACULAR SEGMENTS AND THE UPPER LIP.

Anterior to this metastomal segment, Fig. 116 shows a group of visceral

muscles, _m_1_, and yet again a muco-cartilaginous bar, _sk_1_, but, as

already stated, no tubular muscles. These visceral muscles indicate the

presence in front of the lower lip-segment of one or more segments of the

nature of appendages. The muscles in question (_m_1_) are the muscles of

the upper lip, the skeletal elements form a pair of large bars of

muco-cartilage (_sk_1_), which start from the termination of the trabeculæ,

and pass ventralwards to fuse with the muco-cartilaginous plate of the

lower lip (Figs. 117 and 118). This large bar forms the tentacular ridge on

each side, and gives small projections of muco-cartilage into each

tentacle. In addition to this tentacular bar, a special bar of

muco-cartilage exists for the fused pair of median tentacles, the so-called

tongue, which extends in the middle line along the whole length of the

lower lip, being separated from the muco-cartilaginous plate of the lower

lip by the muscles of the lower lip. This tongue bar of muco-cartilage

joins with the muco-cartilage of the lower lip at its junction with the

thyroid plate, and also with the tentacular bar just before the latter

joins the muco-cartilaginous plate of the lower lip. This arrangement of

the skeletal tissue suggests that the pair of tentacles known as the tongue

stand in a category apart from the rest of the tentacles; a suggestion

which is strongly confirmed by the separate character of its nerve-supply,

as already mentioned.

For three reasons, viz. the separateness both of their nerve-supply and of

their skeletal tissue, and the importance they assume at transformation,

this pair of ventral tentacles must, it seems to me, be put {304}into a

separate category from the rest of the tentacles. On the other hand, the

innervation of the rest of the tentacles by a single nerve which sends off

a branch as it passes each one, together with the concentration of their

skeletal elements into a single bar, with projections into each tentacle,

points directly to the conclusion that these tentacles must be considered

as a group, and not singly.

I suggest that these tentacles are the remains of the ectognaths and

endognaths; the tongue representing the two ectognaths, and the four

tentacles on each side the four pairs of endognaths.

As we see, this method of interpretation attributes segmental value to the

tentacles, a conclusion which is opposed to the general opinion of

morphologists, who regard them as having no special morphological

importance, and certainly no segmental value. On the other hand, the

importance of the pair of ventral tentacles, the 'tongue' of Rathke, which

lie in the mid-line of the lower lip, has been shown by Kaensche, Bujor,

and others, all of whom are unanimous in asserting that at transformation

they are converted into that large and important organ the piston or tongue

of the adult Petromyzon. It is supposed that the rest of the tentacles

vanish at transformation, being absorbed; they appear to me rather to take

part in the formation of the sucking-disc, so that I am strongly inclined

to believe that the whole of the remarkable suctorial apparatus of

Petromyzon is derived from the tentacles of Ammocoetes. In other words, on

my view, a conversion of the prosomatic appendages into a suctorial

apparatus takes place at transformation, just as is frequently the case

among the Arthropoda.

It is to the arrangement of the muscles that we look for evidence of

segmental value. As long as it was possible to look upon these tentacles as

mere sensory feelers round the mouth entrance, it was natural to deny

segmental value to them. Matters are now, however, totally different since

Miss Alcock's discovery of the rudimentary muscles at the base of the

tentacles and their development at transformation. If these muscles

represent some of the appendage muscles belonging to the foremost

prosomatic segments just as the ocular muscles represent the dorso-ventral

somatic muscles of those same segments, then we may expect ultimately to be

able to give as good evidence of segmentation in their case as I have been

able to give in the case of these latter muscles; for the two sets of

muscles are curiously alike, seeing that the eye-muscles do not develop

until {305}transformation, but throughout the Ammocoetes stage remain in

almost as rudimentary a condition as the tentacular muscles.

Another difficulty with respect to the tentacles is the determination of

the number of them, owing to the fact that in addition to what may be

called well-defined tentacles a large number of smaller tactile projections

are found on the surface of the upper lip, as is seen in Fig. 115. In the

very young condition, 7 or 8 mm. in length, it is easier to make sure on

this point. At this stage they may be spoken of as arranged in two groups:

an anterior small group and a posterior larger group. The anterior group

consists of a pair of very small tentacles and a very small median

tentacle, all three situated quite dorsally in the front part of the upper

lip. The posterior group, which is separate from the anterior, consists of

five pairs of much larger tentacles, the most ventral pair in the mid-line

ventrally on the lower lip being fused together to form the large ventral

median tentacle or tongue already mentioned. This pair, according to

Shipley, is markedly larger than the others. There are, therefore, five

conspicuous tentacles on each side, and in front of them a smaller pair and

a small median dorsal one. In the very young condition the accessory

projections above-mentioned are not present, or at all events are not

conspicuous, and the tentacles are also markedly larger in comparison to

the size of the animal than in the older condition, where they have

distinctly dwindled.

This posterior group of five conspicuous tentacles is the one which I

suggest represents the four endognaths and one ectognath. What the

significance of the small anterior group is, I know not. It is possible

that the cheliceræ are represented here, for they are situated distinctly

anterior to the other group; I know, however, of no sign of a markedly

separate innervation to these most dorsal tentacles such as I should have

expected to find if they represented the cheliceræ.

The muscles of the upper lip, which distinctly belong to the visceral and

not to the somatic musculature, form part of the foremost segments, and in

these muscles the tentacular nerve reaches its final destination. From

their innervation, then, they must have belonged to the same appendages as

the tentacles supplied by the tentacular nerve, _i.e._ to the endognaths.

What conclusion can we form as to the probable origin of the upper lip of

Ammocoetes? Since the oral chamber was formed by the forward growth of the

metastoma, _i.e._ the lower lip of Ammocoetes, it follows that the upper

{306}lip is the continuation forwards of the original ventral surface of

such an animal as Limulus or a member of the scorpion group, where there is

no metastoma, and corresponds to the endostoma, as Holm calls it, of

Eurypterus. This termination of the ventral surface in all these animals is

made up of two parts: (1) Of sternites composing the true median ventral

surface of the body, called by Lankester the pro- and meso-sternites; and

(2) of the sterno-coxal processes of the foremost prosomatic appendages,

called in the case of Limulus gnathites, because they are the main agents

in triturating the food previously to its passage into the mouth. In

Limulus, a conjoined pro-mesosternite forms the median ventral wall to

which the sterno-coxal processes are attached on each side, and in Phrynus

and Mygale a well-marked pro-sternite and meso-sternite are present,

forming the posterior limit of the olfactory opening. In Buthus and the

true scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th

prosomatic appendages take part in surrounding the olfactory tubular

passage; in Thelyphonus only the processes of the 2nd pair of prosomatic

appendages play such a part, the pro-sternite not being present (_cf._ Fig.

97).

Seeing, then, what a large share the sterno-coxal processes of one or more

of these prosomatic appendages plays in the formation of this endostoma,

and seeing also that the nerve which supplies the upper lip-muscles in

Ammocoetes is the same as that supplying the tentacles which are attached

to the upper lip, it appears to me more probable than not that the muscles

in question are the vestiges of the sterno-coxal muscles. These muscles

differ markedly in their attachments from the muscles of the lower lip, for

whereas the latter resemble the tergo-coxal group in their extreme dorsal

attachment, the former resemble the sterno-coxal group in their attachment

to what corresponds to the endostoma.

This interpretation of the meaning of the transformation process is in

accordance with all the previous evidence both from the side of the

palæostracan as from the side of the vertebrate, for it signifies that a

dwindling process has taken place in the foremost of the original

prosomatic appendages--the cheliceræ and the endognaths; while, on the

contrary, the ectognath and the metastoma have continued to increase in

importance right into the vertebrate stage. This process is simply a

continuation of what was already going on in the invertebrate stage, for

whereas in Eurypterus and other cases {307}the cheliceræ and endognaths had

dwindled down to mere tentacles, the ectognath was the large swimming

appendage, and the metastoma was on the upward grade from the two

insignificant chilaria of Limulus.

The transformation of these foremost appendages into a suctorial apparatus

is very common among the arthropods, as is seen in the transformation of

the caterpillar into the butterfly, and it is in accordance with the

evidence that the main mass of that suctorial apparatus should be formed

from appendages corresponding to the ectognath and metastoma rather than

from the four endognaths. In all probability the _nucleus masticatorius_ of

the trigeminal nerve with its innervation of the great muscles of

mastication is evidence of the continued development of the musculature of

these two last prosomatic appendages, just as the descending root of the

Vth demonstrates the further disappearance of all that belongs to the

foremost prosomatic appendages. As yet, however, as far as I know, the

musculature of the head-region of Petromyzon has not been brought into line

with that of other vertebrates, and until that comparative study has been

completed it is premature to discuss the exact position of the masticating

muscles of the higher vertebrates.

The analysis of these tentacular segments belonging to the trigeminal nerve

presents greater difficulties than that of any of the other cranial

segments, owing to the deficiency of our knowledge of what occurs at

transformation. Light is required not only on the origin of the new muscles

but also on the origin of the new and elaborate cartilages which are newly

formed at this time.

Miss Alcock has not yet worked out the origin of all these cartilages and

muscles, so that we are not yet in a position to analyze the trigeminal

supply in Petromyzon into its component appendage elements, an analysis

which ought ultimately to enable us to determine from which

appendage-muscles the masticating muscles in the higher vertebrates have

arisen. As far as the muscles are concerned, she gives me the following

information:--

The tongue-nerve supplies in Ammocoetes the rudimentary muscles which pass

laterally from the base of the large ventral tentacle to the wall of the

throat, and even in Ammocoetes must possess some power of moving that

tentacle.

At transformation these muscles proliferate and develop enormously, and

form the bulk of the large basilar muscle which {308}surrounds the throat

ventrally and laterally, and is the most bulky muscle in the suctorial

apparatus.

The velar or mandibular nerve supplies in Ammocoetes the muscles of the

lower lip. In Petromyzon it supplies also the longitudinal muscles of the

tongue. The tongue-cartilage first develops in the region of the median

ventral tentacle, and there the longitudinal tongue-muscles first begin to

develop, not from the rudimentary muscles in the tongue but from those in

the lower lip region.

In Ammocoetes the tentacular nerve supplies the rudimentary muscles in the

tentacles and the muscles of the upper lip. The latter disappear entirely

at transformation, and in Petromyzon the tentacular nerve supplies the

circular, pharyngeal, and annular muscles, which are derived from the

rudimentary tentacular muscles.

For the convenience of my reader I append here a table showing my

conception of the manner in which the endognathal and ectognathal segments

of the Palæostracan are represented in Ammocoetes. It shows well the

uniform manner in which all the individual segmental factors have been

fused together to represent the appearance of a single segment (van Wijhe's

first segment) in the case of the four endognathal segments, but have

retained their individuality in the case of the ectognathal segment.

+----------+----------+---------------------------+----------+----------+

|V. Wijhe's|Eurypterid+-------------+-------------+ Appendage| Skeletal |

| | | | | | |

+----------+----------+-------------+-------------+----------+----------+

| | | | | | |

| | 2} | | | 1 | 1 |

| 1 | 3} | 4 | 4 |Tentacular|Tentacular|

| | 4} | Endognaths | Tentacles | to 4 | bar to 4 |

| | 5} | | | tentacles| tentacles|

| | | | | | |

+----------+----------+-------------+-------------+----------+----------+

| | | | | | |

| 2 | 6 | 1 | 1 | 1 Tongue | 1 Tongue |

| | | | | | |

+----------+----------+-------------+-------------+----------+----------+

+----------+----------+---------+---------+----------+---------+

+----------+----------+---------+---------+----------+---------+

| | | | | | 1 |

| | 2} | 1 Oculo-| Sup. | 1 Pre- |Pituitary|

| 1 | 3} | motor |inf. int.|mandibular| body; |

| | 4} |supplying| rectus | fusion |fusion of|

| | 5} | 4 | and inf.| of 4 | 4 coxal |

+----------+----------+---------+---------+----------+---------+

| 2 | 6 | learis | Sup. | 1 | |

+----------+----------+---------+---------+----------+---------+

{309}THE TUBULAR MUSCLES.

The only musculature innervated by the trigeminal nerve which remains for

further discussion, consists of those peculiar muscles found in the velum,

known by the name of striated tubular muscles. This group of muscles has

already been referred to in Chapter IV., dealing with respiration and the

origin of the heart.

It is a muscular group of extraordinary interest in seeking an answer to

the question of vertebrate ancestry, for, like the thyroid gland, it bears

all the characteristics of a survival from a prevertebrate form, which is

especially well marked in Ammocoetes. I have already suggested in this

chapter that the homologues of these muscles are represented in Limulus by

the veno-pericardial group of muscles. I will now proceed to deal with the

evidence for this suggestion.

The structure of the muscle-fibres is peculiar and very characteristic, so

that wherever they occur they are easily recognized. Each fibre consists of

a core of granular protoplasm, in the centre of which the nuclei are

arranged in a single row. This core is surrounded by a margin of striated

fibrillæ, as is seen in Fig. 122. Such a structure is characteristic of

various forms of striated muscle found in various invertebrates, such as

the muscle-fibre of mollusca. It is, as far as I know, found nowhere in the

vertebrate kingdom, except in Ammocoetes. At transformation these muscles

entirely disappear, becoming fattily degenerated and then absorbed.

[Illustration: FIG. 122.--A TUBULAR MUSCLE-FIBRE OF AMMOCOETES.

A, portion of fibre seen longitudinally; B, transverse section of fibre

(osmic preparation); the black dots are fat-globules.]

For all these reasons they bear the stamp of a survival from a

prevertebrate form. This alone would not make this tissue of any great

importance, but when in addition these muscles are found to be arranged

absolutely segmentally throughout the whole of the branchial region, then

this tissue becomes a clue of the highest importance.

As mentioned in Chapter IV., the segmental muscles of respiration consist

of the adductor muscle and the two constrictor muscles--the {310}striated

constrictor and the tubular constrictor. Of these muscles, both the muscles

possessing ordinary striation are attached to the branchial cartilaginous

skeleton, whereas the tubular constrictors have nothing to do with the

cartilaginous basket-work, but are attached ventrally in the neighbourhood

of the ventral aorta.

These segmental tubular muscles are found also in the velar folds--the

remains of the septum or velum which originally separated the oral from the

respiratory chamber. In the branchial region they act with the other

constrictors as expiratory muscles, forcing the water out of the

respiratory chamber. In the living Ammocoetes, the velar folds on each side

can be seen to move synchronously with the movements of respiration,

contracting at each expiration; they thus close the slit by which the oral

and respiratory chambers communicate, and therefore, in conjunction with

the respiratory muscles, force the water of respiration to flow out through

the gill-slits, as described by Schneider.

These tubular muscles thus form a dorso-ventral system of muscles

essentially connected with respiration; they belong to each one of the

respiratory segments, and are also found in the velum; anterior to this

limit they are not to be found. What, then, are these tubular muscles in

the velar folds? Miss Alcock has worked out their topography by means of

serial sections, and, as already fully explained, has shown that they form

exactly similar dorso-ventral groups, which belong to the two segments

anterior to the purely branchial segments, _i.e._ to the facial or hyoid

segments and the lower lip-segment of the trigeminal nerve. If the velar

folds could be put back into their original position as a septum, then the

hyoid or facial group of tubular muscles would take up exactly the same

position as those belonging to each branchial segment.

The presence of these two so clearly segmental groups of muscles in the

velum--the one belonging to the region of the trigeminal, the other to the

region of the facial--is strong confirmation of my contention that this

septum between the oral and respiratory chambers was caused by the fusion

of the last prosomatic and the first mesosomatic appendages, represented in

Limulus by the chilaria and the operculum.

Yet another clue to the meaning of these muscles is to be found in their

innervation, which is very extraordinary and unexpected. Throughout the

branchial region the striated muscles of each segment {311}are strictly

supplied by the nerve of that segment, and, as already described, each

segment is as carefully mapped out in its innervation as it is in any

arthropod appendage. One exception occurs to this orderly, symmetrical

arrangement: a nerve arises in connection with the facial nerve, and passes

tailwards throughout the whole of the branchial region, giving off a branch

to each segment as it passes. This nerve (_Br. prof._, Fig. 123) is known

by the name of the _ramus branchialis profundus_ of the facial, and its

extraordinary course has always aroused great curiosity in the minds of

vertebrate anatomists. Miss Alcock, by the laborious method of following

its course throughout a complete series of sections, finds that each of the

segmental branches which is given off, passes into the tubular muscles of

that segment (Fig. 124). The tubular muscles which belong to the velum,

_i.e._ those belonging to the lower lip-segment and to the hyoid segments,

receive their innervation from the velar or mandibular nerve, and belong,

therefore, to the trigeminal, not to the facial, system.

[Illustration: FIG. 123.--DIAGRAM SHOWING THE DISTRIBUTION OF THE FACIAL

NERVE.

Motor branches, _red_; sensory branches, blue.]

The evidence presented by these muscles is as follows:--

In the ancestor of the vertebrate there must have existed a segmentally

arranged set of dorso-ventral muscles of peculiar structure, concerned with

respiration, and confined to the mesosomatic segments and to the last

prosomatic segment, yet differing from the other dorso-ventral muscles of

respiration in their innervation and their attachment.

Interpreting these facts with the aid of my theory of the origin of

vertebrates, and remembering that the homologue of the vertebrate ventral

aorta in such a palæostracan as Limulus is the longitudinal {312}venous

sinus, while the opercular and chilarial segments are respectively the

foremost mesosomatic and the last prosomatic segments; they signify that

the palæostracan ancestor must have possessed a separate set of segmental

dorso-ventral muscles confined to the branchial, opercular and chilarial or

metastomal segments, which, on the one hand, were respiratory in function,

and on the other were attached to the longitudinal venous sinus. Further,

these muscles must all have received a nerve-supply from the neuromeres

belonging to the chilarial and opercular segments, an unsymmetrical

arrangement of nerves, on the face of it, very unlikely to occur in an

arthropod.

[Illustration: FIG. 124.--DIAGRAM CONSTRUCTED FROM A SERIES OF TRANSVERSE

SECTIONS THROUGH A BRANCHIAL SEGMENT, SHOWING THE ARRANGEMENT AND RELATIVE

POSITIONS OF THE CARTILAGE, MUSCLES, NERVES, AND BLOOD-VESSELS.

Nerves coloured red are the motor nerves to the branchial muscles. Nerves

coloured blue are the internal sensory nerves to the diaphragms and the

external sensory nerves to the sense-organs of the lateral line system.

_Br. cart._, branchial cartilage; _M. con. str._, striated constrictor

muscles; _M. con. tub._, tubular constrictor muscles; _M. add._, adductor

muscle; _D.A._, dorsal aorta; _V.A._, ventral aorta; _S._, sense-organs on

diaphragm; _n. Lat._, lateral line nerve; _X._, epibranchial ganglia of

vagus; _R. br. prof. VII._, _ramus branchialis profundus_ of facial;

_J.v._, jugular vein; _Ep. pit._, epithelial pit.]

{313}Is this prophecy borne out by the examination of Limulus? In the first

place, these muscles were dorso-ventral and segmental, and, referring back

to Chapter VII., Lankester arranges the segmental dorso-ventral muscles in

three groups: (1) The dorso-ventral somatic muscles; (2) the dorso-ventral

appendage muscles; and (3) the veno-pericardial muscles. Of these the first

group is represented in the vertebrate by the muscles which move the eye,

the second group by the striated constrictor and adductor muscles and the

muscles for the lower lip. There is, then, the possibility of the third

group for this system of tubular muscles.

Looking first at the structure of these muscles as previously described, so

different are they in appearance from the ordinary muscles of Limulus, that

Milne-Edwards, as already stated, called them "brides transparentes," and

did not recognize their muscular character, while Blanchard called them in

the scorpion, "ligaments contractils."

Consider their attachment and their function. They are attached to the

longitudinal sinus, according to Lankester's observation, in such a way

that the muscle-fibres form a hollow cone filled with blood; when they

contract they force this blood towards the gills, and thus act as accessory

or branchial hearts. According to Blanchard, in the scorpion they contract

synchronously with the heart; according to Carlson, in Limulus they

contract with the respiratory muscles. In Ammocoetes, where the respiration

is effected after the fashion of Limulus, not of Scorpio, the tubular

muscles are respiratory in function.

Look at their limits. The veno-pericardial muscles in Limulus are limited

by the extent of the heart, they do not extend beyond the anterior limit of

the heart. In Fig. 70 (p. 176) two of these muscles are seen in front of

the branchial region also attached to the longitudinal venous sinus,

although in front of the gill-region. In Ammocoetes the upper limit of the

tubular muscles is the group found in the velum; this most anterior group

belongs to a region in front of the branchial region--that of the

trigeminal.

Moreover, the supposition that the segmental tubular muscles belong

throughout to the veno-pericardial group gives an adequate reason why they

do not occur in front of the velum; for, as their existence is dependent

upon the longitudinal collecting sinus in Limulus and Scorpio, which is

represented by the ventral aorta in {314}Ammocoetes, they cannot extend

beyond its limits. Now, Dohrn asserts that the ventral aorta terminates in

the spiracular artery, which exists only for a short time; and, in another

place, speaking of this same termination of the ventral aorta, he states:

"Dass je eine vorderste Arterie aus den beiden primären Aesten des Conus

arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der

Mesodermfalte des späteren Velums in die Höhe steigt um in die Aorta der

betreffenden Seite einzumunden." These observations show that the vessel

which in Ammocoetes represents the longitudinal collecting sinus in the

Merostomata does not extend further forwards than the velum, and in

consequence the representatives of the veno-pericardial muscles cannot

extend into the segments anterior to the velum. One of the extraordinary

characteristics of these tubular muscles which distinguishes them from

other muscles, but brings them into close relationship with the

veno-pericardial group, is the manner in which the bundles of muscle-fibres

are always found lying freely in a blood-space; this is clearly seen in the

branchial region, but most strikingly in the velum, the interior of which,

apart from its muco-cartilage, is simply a large lacunar blood-space

traversed by these tubular muscles.

All these reasons point to the same conclusion: the tubular muscles in

Ammocoetes are the successors of the veno-pericardial system of muscles.

If this is so, then this homology ought to throw light on the extraordinary

innervation of these tubular muscles by the _branchialis profundus_ branch

of the facial nerve and the velar branch of the trigeminal. We ought, in

fact, to find in Limulus a nerve arising exclusively from the ganglia

belonging to the chilarial and opercular segments, which, instead of being

confined to those segments, traverses the whole branchial region on each

side, and gives off a branch to each branchial segment; this branch should

supply the veno-pericardial muscle of that side.

Patten and Redenbaugh have traced out the distribution of the peripheral

nerves in Limulus, and have found that from each mesosomatic ganglion a

segmental cardiac nerve arises which passes to the heart and there joins

the cardiac median nerve, or rather the median heart-ganglion, for this

so-called nerve is really a mass of ganglion-cells. In all the branchial

segments the same plan exists, each cardiac nerve belonging to that

neuromere is strictly segmental. {315}Upon reaching the opercular and

chilarial neuromeres an extraordinary exception is found; the cardiac

nerves of these two neuromeres are fused together, run dorsally, and then

form a single nerve called the pericardial nerve, which runs outside the

pericardium along the whole length of the mesosomatic region, and gives off

a branch to each of the cardiac nerves of the branchial neuromeres as it

passes them.

This observation of Patten and Redenbaugh shows that the pericardial nerve

of Limulus agrees with the very nerve postulated by the theory, as far as

concerns its origin from the chilarial and opercular neuromeres, its

remarkable course along the whole branchial region, and its segmental

branches to each branchial segment.

At present the comparison goes no further; there is no evidence available

to show what is the destination of these segmental branches of the

pericardial nerve, and so far all evidence of their having any connection

with the veno-pericardial muscles is wanting. Carlson, at my request,

endeavoured in the living Limulus to see whether stimulation of the

pericardial nerve caused contraction of the veno-pericardial muscles, but

was unable to find any such effect. On the contrary, his experimental work

indicated that each veno-pericardial muscle received its motor supply from

the corresponding mesosomatic ganglion. This is not absolutely conclusive,

for if, as Blanchard asserts in the case of the scorpion, a close

connection exists between the action of these muscles and of the heart, it

is highly probable that their innervation conforms to that of the heart.

Now Carlson has shown that this cardiac nerve from the opercular and

chilarial neuromeres is an inhibitory nerve to the heart, while the

segmental cardiac nerves belonging to the branchial ganglia are the

augmentor nerves of the heart.

His experiments, then, show that the motor nerves of the heart and of the

veno-pericardial muscles run together in the same nerves, but he says

nothing of the inhibitory nerves to the latter muscles. If they exist and

if they are in accordance with those to the heart, then they ought to run

in the pericardial nerve, and would naturally reach the veno-pericardial

muscles by the segmental branches of the pericardial nerve.

Moreover, inhibitory nerves are, in certain cases, curiously associated

with sensory fibres; so that the nerve which corresponds {316}to the

pericardial nerve, viz. the _branchialis profundus_ of the facial, may be

an inhibitory and sensory nerve, and not motor at all. Miss Alcock's

observations are purely histological; no physiological experiments have

been made.

At present, then, it does not seem to me possible to say that Carlson's

experiments have disproved _any_ connection of the pericardial nerve with

the veno-pericardial muscles. We do not know what is the destination of its

segmental branches; they may still supply the veno-pericardial muscles even

if they do not cause them to contract; they certainly do not appear to pass

directly into them, for they pass into the segmental cardiac nerves, and

can only reach the muscles in conjunction with their motor nerves. Such a

course would not be improbable when it is borne in mind how, in the frog,

the augmentor nerves run with the inhibitory along the whole length of the

vagus nerve.

Until further evidence is given both as to the function of the segmental

branches of the pericardial nerve in the Limulus, and of the _branchialis

profundus_ in Ammocoetes, it is impossible, I think, to consider that the

phylogenetic origin of these tubular muscles is as firmly established as is

that of most of the other organs already considered. I must say, my own

bias is strongly in favour of looking upon them as the last trace of the

veno-pericardial system of muscles, a view which is distinctly strengthened

by Carlson's statement that the latter system contracts synchronously with

the respiratory movements, for undoubtedly in Ammocoetes their function is

entirely respiratory. Then again, although at present there is no evidence

to connect the pericardial nerve in Limulus with this veno-pericardial

system of muscles, yet it is extraordinarily significant that in such

animals as Limulus and Ammocoetes, in both of which the mesosomatic or

respiratory region is so markedly segmental, an intrusive nerve should, in

each case, extend through the whole region, giving off branches to each

segment. Still more striking is it that this nerve should arise from the

foremost mesosomatic and the last prosomatic neuromeres in Limulus--the

opercular and chilarial segments--precisely the same neuromeres which give

origin to the corresponding nerve in Ammocoetes, for according to my theory

of the origin of vertebrates, the nerves which supplied the opercular and

metastomal appendages have become the facial nerve and the lower lip-branch

of the trigeminal nerve.

{317}With the formation of the vertebrate heart from the two longitudinal

venous sinuses and the abolition of the dorsal invertebrate heart, the

function of these tubular muscles as branchial hearts was no longer needed,

and their respiratory function alone remained. The last remnant of this is

seen in Ammocoetes, for the ordinary striated muscles were always more

efficient for the respiratory act, and so at transformation the inferior

tubular musculature was got rid of, there being no longer any need for its

continued existence.

THE PALÆOSTOMA, OR OLD MOUTH.

The arrangement of the oral chamber in Ammocoetes is peculiar among

vertebrates, and, upon my theory, is explicable by its comparison with the

accessory oral chamber which apparently existed in Eurypterus. According to

this explanation, the lower lip of the original vertebrate mouth was formed

by the coalescence of the most posterior pair of the prosomatic

appendages--the chilaria; from which it follows that the vertebrate mouth

was not the original mouth, but a new structure due to such a formation of

the lower lip.

It is very suggestive that the direct following out of the original working

hypothesis should lead to this conclusion, for it is universally agreed by

all morphologists that the present mouth is a new formation, and Dohrn has

argued strongly in favour of the mouth being formed by the coalescence of a

pair of gill-slits. Interpret this in the language of my theory, and

immediately we see, as already explained, gill-slits must mean in this

region the spaces between appendages which did not carry gills; the mouth,

therefore, was formed by the coalescence of a pair of appendages to form a

lower lip just as I have pointed out.

Where, then, must we look for the palæostoma, or original mouth? Clearly,

as already suggested, it was situated at the base of the olfactory passage,

and the olfactory passage or nasal tube of Ammocoetes was originally the

tube of the hypophysis, so that the following out of the theory points

directly to the tube of the hypophysis as the place where the palæostoma

must be looked for.

This conclusion is not only not at variance with the opinions of

morphologists, but gives a straightforward, simple explanation why the

palæostoma was situated in the very place where they are most inclined to

locate it. Thus, if we trace the history of the question, {318}we see that

Dohrn's original view of the comparison of the vertebrate and the annelid

led him to the conception that the vertebrate mouth was formed by the

coalescence of a pair of gill-slits, and that the original mouth was

situated somewhere on the dorsal surface and opened into the gut by way of

the infundibulum and the tube of the hypophysis. This, also, was

Cunningham's view as far as the tube of the hypophysis was concerned.

Beard, in 1888, holding the view that the vertebrates were derived from

annelids which had lost their supra-oesophageal ganglia, and that,

therefore, there was no question of an oesophageal tube piercing the

central nervous system of the vertebrate, explained the close connection of

the infundibulum with the hypophysis by the comparison of the tube of the

hypophysis with the annelidan mouth, so that the infundibular or so-called

nervous portion was a special nervous innervation for the original throat,

just as Kleinenberg had shown to be the case in many annelids. Beard

therefore called this opening of the hypophysial tube the old mouth, or

palæostoma. Recently, in 1893, Kupffer has also put forward the view that

the hypophysial opening is the palæostoma. basing this view largely upon

his observations on Ammocoetes and Acipenser.

[Illustration: FIG. 125.--DIAGRAM TO SHOW THE MEETING OF THE FOUR TUBES IN

SUCH A VERTEBRATE AS THE LAMPREY.

_Nc._, neural canal with its infundibular termination; _Nch._, notochord;

_Al._, alimentary canal with its anterior diverticulum; _Hy._, hypophysial

or nasal tube; _Or._, oral chamber closed by septum.]

As is seen in Fig. 125, the position of this palæostoma is a very

suggestive one. At this single point in Ammocoetes, four separate tubes

terminate; here is the end of the notochordal tube, the termination of the

infundibulum, the blind end of the nasal tube or tube {319}of the

hypophysis, and the pre-oral elongation of the alimentary canal.

It is perfectly simple and easy for the olfactory tube to open into any one

of the other three. By opening into the infundibulum it reproduces the

condition of affairs seen in the scorpion; by opening into the gut it

produces the actual condition of things seen in Myxine and other

vertebrates; by opening into the notochordal tube it would produce a

transitional condition between the other two.

The view held by Kupffer is that this nasal tube (tube of the hypophysis)

opened into the anterior diverticulum of the vertebrate gut, and was for

this reason the original mouth-tube; then a new mouth was formed, and this

connection was closed, being subsequently reopened as in Myxine. My view is

that this tube originally opened into the infundibulum, in other words,

into the original gut of the palæostracan ancestor, and was for this reason

the original mouth-tube, in the same sense as the olfactory passage of the

scorpion may be, and often is, called the mouth-tube. When, with the

breaking through of the septum between the oral and respiratory chambers,

the external opening of the oral chamber became a new mouth, the old mouth

was closed but the olfactory tube still remained, owing to the importance

of the sense of smell. Subsequently, as in Myxine and the higher

vertebrates, it opened into the pharynx, and so formed the nose of the

higher vertebrates.

It is not, to my mind, at all improbable that during the transition stage,

between its connection with the old alimentary canal, as in Eurypterus or

the scorpions, and its blind ending, as in Ammocoetes, the nasal tube

opened into the tube of the notochord. This question will be discussed

later on when the probable significance of the notochord is considered.

THE PITUITARY GLAND.

Turning back to the comparison of Fig. 106, B, and Fig. 106, C, which

represent respectively an imaginary sagittal section through an

Eurypterus-like animal and through Ammocoetes at a larval stage, all the

points for comparison mentioned on p. 244 have now been discussed with the

exception of the suggested homology between the coxal glands of the one

animal and the pituitary body of the other.

{320}This latter gland undoubtedly arises posteriorly to the hypophysial

tube, or Rathke's pouch (as it is sometimes called), and, as already

mentioned, is supposed by Kupffer to be formed from the posterior wall of

this pouch. More recently, as pointed out in Haller's paper, Nusbaum, who

has investigated this matter, finds that the glandular hypophysis is not

formed from the walls of Rathke's pouch, but from the tissue of the

rudimentary connection or stalk between the two premandibular cavities,

which becomes closely connected with the posterior wall of Rathke's pouch,

and becoming cut off from the rest of the premandibular cavity on each

side, becomes permanently a part of the 'Hypophysis Anlage.'

The importance of Nusbaum's investigation consists in this, that he derives

the glandular hypophysis from the connecting stalk between the two

premandibular cavities, and therefore from the walls of the ventral

continuation of this cavity on each side.

This may be expressed as follows:--

The coelomic cavity, known as the premandibular cavity, divides into a

dorsal and a ventral part; the walls of the dorsal part give origin to the

somatic muscles belonging to the oculomotor nerve, while the walls of the

ventral part on each side form the connecting stalk between the two

cavities, and give origin to the glandular hypophysis.

Now, as already pointed out, the premandibular cavity is homologous with

the 2nd prosomatic coelomic cavity of Limulus, and this 2nd prosomatic

coelomic cavity divides, according to Kishinouye, into a dorsal and a

ventral part; and, further, the walls of this ventral part form the coxal

gland. Both in the vertebrate, then, and in Limulus, we find a marked

glandular tissue in a corresponding position, and the conclusion is forced

upon us that the glandular hypophysis was originally the coxal gland of the

invertebrate ancestor. As in all other cases already considered, when the

facts of topographical anatomy, of morphology and of embryology, all

combine to the same conclusion as to the derivation of the vertebrate organ

from that of the invertebrate, then there must be also a structural

similarity between the two. What, then, is the nature of the coxal gland in

the scorpions and Limulus? Lankester's paper gives us full information on

this point as far as the scorpion and Limulus are concerned, and he shows

that the coxal gland of Limulus differs markedly from that of Scorpio in

the size of the cells and in the {321}arrangement of the tubes. In Fig.

126, A, I give a picture of a piece of the coxal gland of Limulus taken

from Lankester's paper.

Turning now to the vertebrate, Bela Haller's paper gives us a number of

pictures of the glandular hypophysis from various vertebrates, and he

especially points out the tubular nature of the gland and its

solidification in the course of development in some cases. In Fig. 126, B,

I give his picture of the gland in Ammocoetes.

The striking likeness between Haller's picture and Lankester's picture is

apparent on the face of it, and shows clearly that the histological

structure of the glands in the two cases confirms the deductions drawn from

their anatomical and morphological positions.

[Illustration: FIG. 126.--A, SECTION OF COXAL GLAND OF LIMULUS (from

LANKESTER); B, SECTION OF PITUITARY BODY OF AMMOCOETES (from BELA HALLER).

_n.a._, termination of nasal passage.]

The sequence of events which gave rise to the pituitary body of the

vertebrate was in all probability somewhat as follows:--

Starting with the excretory glands of the Phyllopoda, known as

shell-glands, which existed almost certainly in the phyllopod Trilobite, we

pass to the coxal gland of the Merostomata. Judging from Limulus, these

were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor

appendages. When these appendages became reduced in size and purely tactile

they were compressed and concentrated round the mouth region, forming the

endognaths of the Merostomata; as a necessary consequence of the

concentration of the coxæ of the endognaths, the coxal gland also became

concentrated, {322}and took up a situation close against the pharynx, as

represented in Fig. 106, B. When, then, the old mouth closed, and the

pharynx became the _saccus vasculosus_, the coxal gland remained in close

contact with the _saccus vasculosus_, and became the pituitary body, thus

giving the reason why there is always so close a connection between the

pituitary body and the infundibular region.

Whatever was the condition of the digestive tracts at the transition stage

between the arthropod and the vertebrate, the original mouth-opening at the

base of the olfactory tube was ultimately closed. The method of its closure

was exceedingly simple and evident. The membranous cranium was in process

of formation by the extension of the plastron laterally and dorsally; a

slight growth of the same tissue in the region of the mouth would suffice

to close it and thus separate the infundibulum from the olfactory tube. As

evidence that such was the method of closure, it is instructive to see how

in Ammocoetes the glandular tissue of the pituitary body is embedded in and

mixed up with the tissue of this cranial wall; how the termination of the

nasal tube is embedded in this same thickened mass of the cranial

wall--how, in fact, both coxal gland and olfactory tube have become

involved in the growth of the tissue of the plastron, by means of which the

mouth was closed.

I have now passed in review the nature of the evidence which justifies a

comparison between the segments supplied by the cranial nerves of the

vertebrate and the prosomatic and mesosomatic segments of the palæostracan.

For the convenience of my readers I have put these conclusions into tabular

form (see p. 323), for all the segments as far as that supplied by the

glossopharyngeal nerves. In both vertebrate and invertebrate this is a

fixed position, for in the former, however variable may be the number of

branchial segments which the vagus supplies, the second branchial segment

is always supplied by a separate nerve, the glossopharyngeal, and in the

latter, though the number of segments bearing branchiæ varies, the minimum

number of such segments (as seen in the Pedipalpi) is never less than two.

{323}TABLE OF COMPARISON OF CORRESPONDING SEGMENTS IN THE EURYPTERIDS AND

IN AMMOCOETES (_i.e._ IN CEPHALASPIDS).

Key:

So. = Supra-oesophageal.

Si. = Supra-infundibular.

Io. = Infra-oesophageal.

Ii. = Infra-infundibular.

Ps. = Prosomatic.

Ms. = Mesosomatic.

+---+--------------------------------------------------------+

| | Median Eyes. |

| +--------------------------------------------------------+

|So.| Lateral Eyes. |

| +--------------------------------------------------------+

| | Camerostome. |

+---+--------------------------------------------------------+

| | Invertebrate (Limulus or Eurypterid). |

+---+---+---------+-----------------------------+------------+

| | |Segments.+-------------+---------------+ Cavities. |

| | +---------+-------------+---------------+------------+

| | | 1 | Cheliceræ | Cheliceræ | 1 |

| | +---------+-------------+---------------+------------+

| | | 2 |1st Locomotor|} | |

| | +---------+-------------+} | 2 |

| | | 3 | 2nd " |} |Ventral part|

| |Ps.+---------+-------------+}4 Endognaths | forms coxal|

| | | 4 | 3rd " |} | gland. |

| | +---------+-------------+} | |

| | | 5 | 4th " |} | |

| | +---------+-------------+---------------+------------+

|Io.| | 6 | 5th " | Ectognath | 3 |

| | | | | | |

| | +---------+-------------+---------------+------------+

| | | 7 | Chilaria | Metastoma | 4 |

| | | | | | |

| +---+---------+-------------+---------------+------------+

| | | 8 | Operculum |Genital } | 5 |

| | |---------+-------------+ }Oper-+------------+

| | | 9 |1st Branchial| }culum| 6 |

| | | | |Branchial} | |

| |Ms.+---------+-------------+---------------+------------+

| | | 10 | 2nd " | 2nd Branchial | 7 |

+---+---+---------+-------------+---------------+------------+

+-----------------------------------------------------+------+

| Pineal Eyes. | |

+-----------------------------------------------------+ |

| Lateral Eyes. | Si. |

+-----------------------------------------------------+ |

| Olfactory Organ. | |

+-----------------------------------------------------+------+

| Vertebrate (Ammocoetes or Cephalaspid). | |

+-------------+---------------+-------------+---------+------+

+-------------+---------------+-------------+---------+ |

| ... | ... | ... | ... | |

+-------------+---------------+-------------+---------+ |

| | | | | |

| | V | Muscles | | |

| | | | | |

+-------------+---------------+-------------+---------+ |

| Tongue | V | Sup.oblique | IV | Ii. |

+-------------+---------------+-------------+---------+ |

| Lower lip | V | | | |

+-------------+---------------+-------------+---------+ |

| Thyroid |} | | | |

+-------------+} VII | Ext. rectus | VI | |

| Hyoid or |} |Retract oculi| | |

|1st Branchial|} | | | |

+-------------+---------------+-------------+---------+ |

|2nd Branchial| IX | ... | ... | |

+-------------+---------------+-------------+---------+------+

+-------------------------+------+

| Pineal Nerve. | |

+-------------------------+ |

| II | Si. |

+-------------------------+ |

| I | |

+-------------------------+------+

| Vertebrate. | |

+-------------+-----------+------+

| Coelomic |V. Wijhe's | |

| Cavities. | Segments. | |

| | | |

+-------------+-----------+ |

| Anterior. | ... | |

+-------------+-----------+ |

| Premandi- | | |

| bular | | |

| Ventral | | |

| part forms | 1 | |

| pituitary | | |

| body. | | |

| pituitary | | |

+-------------+-----------+ |

| Mandibular |} | Ii. |

| |} | |

+-------------+} 2 | |

| Mandibular |} | |

| |} | |

+-------------+-----------+ |

| Hyoid_1 | 3 | |

+-------------+-----------+ |

| Hyoid_2 | 4 | |

| | | |

+-------------+-----------+ |

|2nd Branchial| 5 | |

+-------------+-----------+------+

{324}SUMMARY.

The general consideration of the evidence of the number of segments, and

their nature in the pro-otic region of the vertebrate, as given in the

last chapter, is not incompatible with the view that the trigeminal nerve

originally supplied seven appendages, which appendages did not carry

branchiæ, but were originally used for purposes of locomotion as well as

of mastication.

Such appendages clearly no longer exist in the higher vertebrates, the

muscles of mastication only remaining; but in the earliest fish-forms

they must have existed, as, indeed, is seen in Pterichthys and

Bothriolepis. Judging from all the previous evidence some signs of their

existence may reasonably be expected still to remain in Ammocoetes. Such

is indeed the case.

In the adult Petromyzon the trigeminal nerve innervates specially a

massive suctorial apparatus, by means of which it holds on to other

fishes, or to stones in the bottom of the stream. There is here no

apparent sign of appendages. Very great, however, is the difference in

the oral chamber of Ammocoetes; here there is no sign of any suctorial

apparatus, but instead, a system of tentacles, together with the remains

of the septum or velum, which originally closed off the oral from the

respiratory chamber. These tentacles are the last remnants of the

original foremost prosomatic appendages of the palæostracan ancestor.

Like the lateral eyes they do not develop until the transformation comes,

but during the whole larval condition their musculature remains in an

embryonic condition, and then from these embryonic muscles the whole

massive musculature of the suctorial apparatus develops; a sucking

apparatus derived from the modification of appendages, as so frequently

occurs in the arthropods.

The study of Ammocoetes indicates that the velum and lower lip correspond

to the metastoma of the Eurypterid, _i.e._ the chilaria of Limulus, while

the large ventral pair of tentacles, called the tongue, correspond to the

ectognaths of the Eurypterids, and probably to the oar-like appendages of

Pterichthys and Bothriolepis. From these two splanchnic segments the

suctorial apparatus in the main arises; the motor supply of these two

segments forms the mass of the trigeminal nerve-supply, and the nerves

supplying them, the velar nerve and the tongue-nerve, are markedly

separate from the rest of the trigeminal nerve.

The rest of the tentacles present much less the sign of independent

segments. In their nerves, their muco-cartilaginous skeleton, and their

rudimentary muscles, they indicate a concentration and amalgamation, such

as might be expected from the concentrated endognaths. The continuation

of the dwindling process, already initiated in the Eurypterid, would

easily result in the tentacles of Ammocoetes.

The nasal tube of Ammocoetes, which originates in the hypophysial tube,

corresponds absolutely in position and in its original structure, to the

olfactory tube of a scorpion-like animal. From this homology two

conclusions of importance follow: (1) the old mouth, or palæostoma, of

the vertebrate was situated at the end of this tube, therefore, at the

termination of the infundibulum; (2) the upper lip, which by its growth,

brings the olfactory tube from a ventral to a dorsal position, was

originally formed by the foremost sternites or endostoma, or else by the

sterno-coxal processes of the second pair of prosomatic appendages of the

palæostracan ancestor.

In strict accordance with the rest of the comparisons made in this

region, the pituitary body shows by similarity of structure, as well as

of position, that it arose from the coxal glands, which were situated at

the base of the four endognaths.

{325}One after another, when once the clue has been found, all these

mysterious organs of the vertebrate, such as the pituitary and thyroid

glands, fall harmoniously into their place as the remnants of

corresponding important organs in the palæostraca.

Yet another clue is afforded by the tubular muscles of Ammocoetes, that

strange set of non-vertebrate striated muscles, which are so markedly

arranged in a segmental manner, which disappear at transformation, and

are never found in any of the higher vertebrates, for the limits of their

distribution correspond to the veno-pericardial muscles of Limulus.

Their nerve-supply in Ammocoetes is most extraordinary; for, although

they are segmentally arranged throughout the whole respiratory region,

which is segmentally supplied by the VIIth, IXth, and Xth nerves, and are

found in front of this region only in one segment, that of the lower lip,

which is supplied by the velar branch of the Vth nerve, yet they are not

supplied segmentally, but only by the velar nerve and a branch of the

VIIth, the _ramus branchialis profundus_. This latter nerve extends

throughout the respiratory region, and gives off segmental branches to

supply these muscles.

It is also a curious coincidence that in such a markedly segmented animal

as Limulus, a nerve--the pericardial nerve--which arises from the nerves

of the chilarial and opercular segments, should pass along the whole

respiratory region and give off branches to each mesosomatic segment. It

is strange, to say the least of it, that the chilarial or metastomal and

the opercular segments of Limulus should, on the theory advocated in this

book, correspond to the lower lip and hyoid segments of the vertebrate.

At present the homology suggested is not complete, for there is no

evidence as yet that the veno-pericardial muscles have anything to do

with the pericardial nerve.

{326}CHAPTER X

_THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE

OSTRACODERMATA_

The nose of the Osteostraci.--Comparison of head-shield of Ammocoetes and

of Cephalaspis.--Ammocoetes the only living representative of these

ancient fishes.--Formation of cranium.--Closure of old mouth.--Rohon's

primordial cranium.--Primordial cranium of Phrynus and

Galeodes.--Summary.

The shifting of the orifice of the olfactory passage, which led to the old

mouth, from the ventral to the dorsal side, as seen in the transformation

of the ventrally situated hypophysial tube of the young Ammocoetes, to the

dorsally situated nasal tube of the full-grown Ammocoetes, affords one of

the most important clues in the whole of this story of the origin of

vertebrates; for, if Ammocoetes is the nearest living representative of the

first-formed fishes, then we ought to expect to find that the dorsal

head-shield of such fishes is differentiated from that of the contemporary

Palæostraca by the presence of a median frontal opening anterior to the

eyes. Conversely, if such median nasal orifice is found to be a marked

characteristic of the group, in combination with lateral and median eyes,

as in Ammocoetes, then we have strong reasons for interpreting these

head-shields by reference to the head of Ammocoetes.

The oldest known fishes belong to a large group of strange forms which

inhabited the Silurian and Devonian seas, classed together by Smith

Woodward under the name of Ostracodermi. These are divided into three

orders: (1) the Heterostraci, including one family, the Pteraspidæ, to

which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into

two families, the Cephalaspidæ and Tremataspidæ, which include Cephalaspis,

Eukeraspis, Auchenaspis or Thyestes, and Tremataspis; and (3) the

Antiarcha, with one family, the Astrolepidæ, including Astrolepis,

Pterichthys, and Bothriolepis. {327}Of these, the first two orders belong

to the Upper Silurian, while the third is Devonian.

THE DORSAL HEAD-SHIELD OF THE OSTEOSTRACI.

Of the three orders above-named, the Heterostraci and Osteostraci are the

oldest, and among them the Cephalaspidæ have afforded the most numerous and

best worked-out specimens. At Rootziküll, in the island of Oesel, the form

known as _Thyestes (Auchenaspis) verrucosus_ is especially plentiful, being

found thickly present in among the masses of Eurypterid remains, which give

the name to the deposit. Of late years this species has been especially

worked at by Rohon, and many beautiful specimens have been figured by him,

so that a considerable advance has been made in our knowledge since Pander,

Eichwald, Huxley, Lankester, and Schmidt studied these most interesting

primitive forms.

All observers agree that the head-region of these fishes was covered by a

dorsal and ventral head-shield, while the body-region was in most cases

unknown, or, as in Eichwald's specimens, and in the specimens figured in

Lankester and Smith Woodward's memoirs, was made up of segments which were

not vertebral in character, but formed an aponeurotic skeleton, being the

hardened aponeuroses between the body-muscles. This body-skeleton, which

possesses its exact counterpart in Ammocoetes, will be considered more

fully when I discuss the origin of the spinal region of the vertebrate.

Of the two head-shields, ventral and dorsal, the latter is best known and

characterizes the group. It consists of a dorsal plate, with characteristic

horns, which in _Thyestes verrucosus_ (Fig. 128), as described by Rohon, is

composed of two parts, a frontal part and an occipital part (_occ._), the

occipital part being composed of segments, and possessing a median

ridge--the _crista occipitalis_. In Lankester's memoir and in Smith

Woodward's catalogue, a large number of known forms are described and

delineated, and we may perhaps say that in some of the forms, such as

_Eukeraspis pustuliferus_ (Fig. 127, B), the frontal part of the shield

only is capable of preservation as a fossil, while in Cephalaspis (Fig.

127, A) not only the frontal part but a portion of the occipital region is

preserved, the latter being small in extent when compared with the

occipital region of Auchenaspis (Thyestes). Finally, in Tremataspis and

Didymaspis, the whole of both frontal {328}and occipital region is capable

of preservation, the line of demarcation between these two regions being

well marked in the latter species.

[Illustration: FIG. 127.--A, DORSAL HEAD-SHIELD OF CEPHALASPIS (from

LANKESTER); B, DORSAL HEAD-SHIELD OF KERASPIS (from LANKESTER).]

In the best preserved specimens of all this group of fishes a frontal

median orifice is always present; it appears in some specimens obscurely

partially divided into two parts. Perhaps the best specimen of all was

obtained by Rohon at Rootziküll, and is thus described by him:--

The frontal part of the dorsal head-plate carried (Fig. 128) the two orbits

for the lateral eyes (_l.e._), a marked frontal organ (_fro._), and a

median depression (_gl._), to which he gives the name parietal organ. The

occipital part (_occ._) was clearly segmented, and carried, he thinks, the

branchiæ. I reproduce Rohon's figure of the frontal organ in Thyestes (Fig.

129); he describes it as a deeply sunk pit, divided in the middle by a

slit, which leads deeper in, he supposes, towards the central nervous

system.

[Illustration: FIG. 128.--DORSAL HEAD-SHIELD OF _Thyestes (Auchenaspis)

verrucosus_. (From ROHON.)

_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate

over brain; _Occ._, occipital region.]

{329}A similar organ was described by Schmidt in Tremataspis, and

considered by him to be a median nose. Such also is the view of Jaekel, who

points out that a median pineal eye exists between the two lateral eyes in

this animal, as in all other of these ancient fishes, so that this frontal

organ does not, as Patten thinks, represent the pineal eye. The whole of

this group of fishes, then, is characterized by the following striking

characteristics:--

1. Two well-marked lateral eyes near the middle line.

2. Between the lateral eyes, well-marked median eyes, very small.

3. In front of the eye-region a median orifice, single.

In addition, behind the eye-region a median plate is always found,

frequently different in structure to the rest of the head-shield, being

harder in texture--the so-called post-orbital plate.

[Illustration: FIG. 129.--NARIAL OPENING AND LATERAL ORBITS OF _Thyestes

Verrucosus_. (From ROHON.)]

STRUCTURE OF HEAD-SHIELD OF CEPHALASPIS COMPARED WITH THAT OF AMMOCOETES.

What is the structure of this head-shield? It has been spoken of as formed

of bone because it possesses cells, being thus unlike the layers of chitin,

which are formed by underlying cells but are not themselves cellular. At

the same time, it is recognized on all sides that it has no resemblance to

bone-structure as seen in fossil remains of higher vertebrates. The latest

and best figure of the structure of this so-called bone is given in Rohon's

paper already referred to. It is, so he describes, clearly composed of

fibrillæ and star-shaped cells, arranged more or less in regular layers,

with other sets of similar cells and fibrillæ arranged at right angles to

the first set, or at varying angles. The groundwork of this tissue, in

which these cells and fibrils are embedded, contained calcium salts, and so

the whole tissue was preserved. In places, spaces are found in it, in the

deepest layer large medullary spaces; more superficially, ramifying spaces

which he considers to be vascular, and calls Haversian canals; the

{330}star-like cells, however, are not arranged concentrically around these

spaces, as in true Haversian canals.

This structure is therefore a calcareous infiltration of a tissue with

cells in it. Where is there anything like it?

As soon as I saw Rohon's picture (Fig. 130), I was astounded at its

startling resemblance to the structure of muco-cartilage as is seen in Fig.

131, taken from Ammocoetes. If such muco-cartilage were infiltrated with

lime salts, then the muco-cartilaginous skeleton of Ammocoetes would be

preserved in the fossil condition, and be comparable with that of

Cephalaspis, etc.

[Illustration: FIG. 130.--SECTION OF A HEAD-PLATE OF A CEPHALASPID. (From

ROHON.)]

[Illustration: FIG. 131.--SECTION OF MUCO-CARTILAGE FROM DORSAL HEAD-PLATE

OF AMMOCOETES.]

The whole structure is clearly remarkably like Rohon's picture of a section

of the head-plate of a Cephalaspid (Fig. 130). In the latter case the

matrix contains calcium salts, in the former it is composed of the peculiar

homogeneous mucoid tissue which stains so characteristically with thionin.

With respect to this calcification, it is instructive to recall the

calcification in the interior of the branchial cartilages of Limulus, as

described in Chapter III., for this example shows how easy it is to obtain

a calcification in this chondro-mucoid material. With respect to the

medullary spaces and smaller spaces in this tissue, as described by Rohon,

I would venture to suggest that they need not all necessarily indicate

blood-vessels, for similar spaces would appear in the head-shield of

Ammocoetes if its muco-cartilage alone {331}were preserved. Of these, some

would indicate the position of blood-vessels, such, for instance, as of the

external carotid which traverses this structure; but the largest and most

internal spaces, resembling Rohon's medullary spaces, would represent

muscles, being filled up with bundles of the upper lip-muscles.

THE MUCO-CARTILAGINOUS HEAD-SHIELD OF AMMOCOETES.

The resemblance between the structure of the head-shield of Thyestes and

the muco-cartilage of Ammocoetes, is most valuable, for muco-cartilage is

unique, occurs in no other vertebrate, and every trace of it vanishes at

transformation; it is essentially a characteristic of the larval form, and

must, therefore, in accordance with all that has gone before, be the

remnant of an ancestral skeletal tissue. The whole story deduced from the

study of Ammocoetes would be incomplete without some idea of the meaning of

this tissue. So also, as already mentioned, the skeleton of Ammocoetes is

incomplete without taking this tissue into account. It is confined entirely

to the head-region; no trace of it exists posteriorly to the branchial

basket-work. It consists essentially of dorsal and ventral head-shields,

connected together by the tentacular, metastomal, and thyroid bars, as

already described. The ventral shield forms the muco-cartilaginous plate of

the lower lip and the plate over the thyroid gland, so that the skeleton

ventrally is represented by Fig. 118, B, which shows how the cartilaginous

bars of the branchial basket-work are separated from each other by this

thyroid plate. At transformation, with the disappearance of this

muco-cartilaginous plate, the bars come together in the middle line, as in

the more posterior portion of the branchial basket-work.

The dorsal head-shield of muco-cartilage covers over the upper lip, sends a

median prolongation over the median pineal eyes and a lateral prolongation

on each side as far as the auditory capsules, giving the shape of the

head-shield of muco-cartilage, as in Fig. 118, C.

Not only then is the structure of the head-shield of a Cephalaspid

remarkably like the muco-cartilage of Ammocoetes, but also its general

distribution strangely resembles that of the Ammocoetes muco-cartilage.

Now, these head-shields in the Cephalaspidæ and Tremataspidæ {332}vary very

much in shape, as is seen by the comparison of Tremataspis and Auchenaspis

with Cephalaspis and Eukeraspis, and yet, undoubtedly, all these forms

belong to a single group, the Osteostraci.

The conception that Ammocoetes is the solitary living form allied to this

group affords a clue to the meaning of this variation of shape, which

appears to me to be possible, if not indeed probable. There is a certain

amount of evidence given in the development of Ammocoetes which indicates

that the branchial region of its ancestors was covered with plates of

muco-cartilage as well as the prosomatic region.

The evidence is as follows:--

The somatic muscles of Ammocoetes form a continuous longitudinal sheet of

muscles along the length of the body, which are divided up by connective

tissue bands into a series of imperfect segments or myotomes. This simple

muscular sheet can be dissected off along the whole of the head-region of

the animal, with the exception of the most anterior part, without

interfering with the attachments or arrangements of the splanchnic muscular

system in the least. The reason why this separation can be so easily

effected is to be found in the fact that the two sets of muscles are not

attached to the same fascia. The sheet of fascia to which the somatic

muscles are attached is separated from the fascia which encloses the

branchial cavity by a space (_cf._ Figs. 63 and 64) filled with

blood-spaces and cells containing fat, in which space is also situated the

cartilaginous branchial basket-work. These branchial bars are closely

connected with the branchial sheet of fascia, and have no connection with

the somatic fascia, their perichondrium forming part of the former sheet.

Upon examination, this space is seen to be mainly vascular, the

blood-spaces being large and frequently marked with pigment; but it also

possesses a tissue of its own, recognized as fat-tissue by all observers.

The peculiarity of the cells of this tissue is their arrangement; they are

elongated cells arranged at right angles to the plates of fascia, just as

the fibres of the muco-cartilage are largely arranged at right angles to

their limiting plates of perichondrium. These cells do not necessarily

contain fat; and when they do, the fat is found in the centre of each cell,

and does not push the protoplasm of the cell to the periphery, as in

ordinary fat cells.

{333}In Fig. 132, B, I give a specimen of this tissue stained by osmic

acid; in Fig. 132, A, I give a drawing of ordinary muco-cartilage taken

from the plate of the lower lip; and in Fig. 133, A, a modification of the

muco-cartilage taken from the velum, which shows the formation of a tissue

intermediate between ordinary muco-cartilage and this branchial fat-tissue.

Further, in fully-grown specimens of Ammocoetes, in the region of undoubted

muco-cartilage, a fatty degeneration of the cells frequently appears,

together with an increase in the blood spaces,--the precursor, in fact, of

the great change which overtakes this tissue soon afterwards, at the time

of transformation, when it is invaded by blood, and swept away, except in

those places where new cartilage is formed. I conclude, then, that the

tissue of this vascular space was originally muco-cartilage, which has

degenerated during the life of the Ammocoetes. The fact that in most cases

undoubted muco-cartilage is to be found here and there in this space, is

strong confirmation of the truth of this conclusion.

[Illustration: FIG. 132.--A, MUCO-CARTILAGE OF LOWER LIP (_Mc._); _m.ph._,

muscle of lower lip; _m.sm._, somatic muscle; _Cor._, laminated layer of

skin. B, DEGENERATED MUCO-CARTILAGE OF BRANCHIAL REGION. _F._, fat layer;

_P._, pigment; _Bl._, blood-space; _N._, somatic nerve; _m.br._, branchial

muscle; _m.sm._, somatic muscle.]

If this conclusion is correct, we may expect that it would be confirmed by

the embryological history of the tissue, and we ought to find that in much

younger stages a homogeneous tissue of the same nature as muco-cartilage

fills up the spaces in the branchial {334}region, where in the Ammocoetes

only blood and fat-containing cells are present. For this purpose Shipley

kindly allowed me to examine his series of sections through the embryo at

various ages. These specimens are very instructive, especially those

stained by osmic acid, which preserves the natural thickness of this space

better than other staining methods. At an age when the branchial cartilages

are seen to be formed, when no fat-cells are present, a distinctive tissue

(Fig. 133, B) is plainly visible in the velum and at the base of the

tentacles, in the very position where in the more advanced Ammocoetes

muco-cartilage exists. Taking, then, this tissue as our guide, the

specimens show that the space between the skin and the visceral muscles in

which the cartilaginous basket-work lies is filled with a similar material.

At this stage a sheet of embryonic tissue occupies the position where,

later on, blood-spaces and fat-cells are found, and this tissue resembles

that seen in the velum and other places where muco-cartilage is afterwards

found.

[Illustration: FIG. 133.--A, MUCO-CARTILAGE OF VELUM; B, EMBRYONIC

MUCO-CARTILAGE OF TENTACULAR BAR.]

I conclude, therefore, that originally the branchial or mesosomatic region

was covered with a dorsal plate of muco-cartilage, which carried on its

under surface the dorsal part of the branchial basket-work, and sprang from

the central core of skeletogenous tissue around the notochord; this plate

was separated from the plate which covered this region ventrally by the

lateral grove in which the gill-slits are situated. The ventral plate

carried on its under surface the ventral part of the branchial basket-work,

and was originally continuous with the plate over the thyroid gland.

{335}[Illustration: FIG. 134.--SKELETON OF HEAD-REGION OF AMMOCOETES. A,

LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW.

Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_.

_sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na.

cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._,

notochord.]

{336}In Fig. 134, A, B, C, the cranial skeleton of Ammocoetes is

represented from the dorsal, ventral, and lateral aspects. The

muco-cartilage is coloured red, the branchial or soft cartilage blue, and

the hard cartilage purple. The degenerated muco-cartilage of the branchial

region is represented as an uncoloured plate, on which the branchial

basket-work stands in relief. If it were restored to its original condition

of muco-cartilage, it would represent a uniform plate, on the _under_

surface of which the basket-work would be situated; and if it were

calcified and made solid, the branchial basket-work would not show at all

in these figures.

Is it possible to find the reason why this skeletal covering has

degenerated so early before transformation, and why the thyroid plate

remains intact until transformation? We see that all that part which has

degenerated is covered over by the somatic muscles,--by, in fact, muscles

which, being innervated by the foremost spinal nerves, belong naturally to

the region immediately following the branchial. I suggest, therefore, that

the original skeletal covering of muco-cartilage has remained intact only

where it has not been invaded and covered over by somatic muscles, but has

been invaded by blood and undergone the same kind of degenerative change as

overtakes the great mass of this tissue at transformation wherever the

somatic muscles have overgrown it.

The covering somatic muscles in the branchial region form a dorsal and

ventral group, of which the latter is formed in the embryo much later than

the former, the line of separation between the two groups being the lateral

groove, with its row of branchial openings. This groove ends at the first

branchial opening, but the ventral and dorsal somatic muscles continue

further headwards. It is instructive to see that, although the lateral

groove terminates, the separation between the two groups of muscles is

still marked out by a ridge of muco-cartilage, represented in Fig. 134, A,

which terminates anteriorly in the opercular bar.

Passing now to the prosomatic region, we find that here, too, the

muco-cartilaginous external covering is divisible into a dorsal and a

ventral head-plate, the ventral head-plate being the plate of the lower

lip, and the dorsal head-plate the plate of muco-cartilage over the front

part of the head. The staining reaction with thionin maps out this dorsal

head-plate in a most beautiful manner, and shows that the whole of the

upper lip-region in front of the nasal orifice is one large plate of

muco-cartilage, obscured largely by the invasion of the crossing muscles of

the upper lip, but left pure and uninvaded all around the nasal orifice,

and where the upper and lower lips come together. In addition to this

foremost plate, a median tongue of muco-cartilage covers over the pineal

eye and fills up the {337}median depression between the two median dorsal

somatic muscles. Also, two lateral cornua pass caudalwards from the main

frontal mass of muco-cartilage over the lateral eyes, forming the

well-known wedge which separates the dorsal and lateral portions of the

dorso-lateral somatic muscle. In fact, similarly to what we find in the

branchial region, the muco-cartilaginous covering can be traced with

greater or less completeness only in those parts which are not covered by

somatic muscles.

In Fig. 134, A, B, C, this striking muco-cartilaginous head-shield, both

dorsal and ventral, is shown. Seeing that the upper lip wraps round the

lower one on each side, and that this most ventral edge of the upper lip

contains muco-cartilage, as is seen in Fig. 117, the dorsal head-shield of

muco-cartilage ought, strictly speaking, to extend more ventrally in the

drawings. I have curtailed it in order not to interfere with the

representation of the lower lip and tentacular muco-cartilages.

From what has been said, it follows that the past history of the skeletal

covering of the whole head-region of Ammocoetes, both frontal and

occipital, can be conjectured by means of the ontogenetic history of the

foremost myomeres.

Dohrn and all other observers are agreed that during the development of

this animal a striking forward growth of the foremost somatic myomeres

takes place, so that, as Dohrn puts it, the body-musculature has extended

forwards over the gill-region, and at the same time the gill-region has

extended backwards. It is therefore probable that in the ancestral form the

myotomes, innervated by the first spinal nerves, immediately succeeded the

branchial region. Judging from Ammocoetes, the forward growth was at first

confined to the dorsal region, and therefore invaded the dorsal head-plate,

the ventral musculature being distinctly a later growth. With respect to

this dorsal part of the myotomes, the first myotome is originally situated

some distance behind the auditory capsule, and then grows forward towards

the nasal opening; the lateral part, according to Hatschek, grows forward

more quickly than the dorsal part, and splits itself above and below the

eye into a dorso-lateral part, which extends up to the olfactory capsule,

and a ventro-lateral part (_m. lateralis capitis_ anterior, superior, and

inferior), thus giving rise to the characteristic appearance of the

muco-cartilaginous head-shield of Ammocoetes.

According, then, to the extent of the growth of these somatic {338}muscles,

the shape of the muco-cartilaginous head-shield will vary, and if it were

calcified and then fossilized we should obtain fossil head-shields of

widely differing configuration, although such fossils might be closely

allied to each other. This is just what is found in this group. Let the

muco-cartilage extend over the whole of the branchial region of Ammocoetes,

the resulting head-shield would be as in Fig. 135, A; the branchial bars

below the muco-cartilaginous shield might or might not be evident, and the

line between the branchial and the trigeminal region might or might not be

indicated. Such a head-shield would closely resemble those of Didymaspis

and Tremataspis respectively. Now suppose the somatic musculature to

encroach slightly on the branchial region and also laterally to the end of

the anterior branchial region, then we should obtain a shape resembling

that of Thyestes (Fig. 135, B). Continue the same process further, the

lateral muscle always encroaching further than the median masses, until the

whole or nearly the whole branchial region is invested, and we get the

head-shield of Cephalaspis (Fig. 135, C); further still, that of Keraspis,

and yet still further, that of Ammocoetes (Fig. 135, D).

[Illustration: FIG. 135.--DIAGRAMS TO SHOW THE DIFFERENT SHAPES OF

HEAD-SHIELDS DUE TO THE FORWARD GROWTH OF THE SOMATIC MUSCULATURE.

A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammocoetes.]

So close is this similarity, from the comparative point of view, between

the dorsal head-shield of the Osteostraci and the dorsal cephalic region of

Ammocoetes that it justifies us in taking Ammocoetes as the nearest living

representative of such types; it is justifiable, therefore, to interpret by

means of Ammocoetes the position of other organs in these forms. First and

foremost is the hard plate {339}known as the post-orbital plate, so

invariably found. In Fig. 134, C, I have inserted (_cr._) the position of

the membranous cranium of Ammocoetes, and it is immediately evident that

the primordial cranium of the Osteostraci must occupy the exact position

indicated by this median hard plate. For this very reason this median plate

would be harder than the rest in order to afford a better protection to the

brain underneath. This plate, because of its position, may well receive the

same name as the similar plate in the trilobite and various palæostracans

and be called the glabellum.

EVIDENCE OF SEGMENTATION IN THE HEAD-SHIELD--FORMATION OF CRANIUM.

We may thus conceive the position of the nose, lateral eyes, median eyes,

and cranium in these old fishes. In addition, other indications of a

segmentation in this head-region have been found. The most striking of all

the specimens hitherto discovered are some of _Thyestes verrucosus_,

discovered by Rohon, in which the dorsal shield has been removed, and so we

are able to see what that dorsal shield covered.

In Fig. 136, I reproduce his drawing of one of his specimens from the

dorsal and lateral aspects. These drawings show that the frontal part of

the shield covered a markedly segmented part of the animal; five distinct

segments are visible apart from the median most anterior region. This

segmented region is entirely confined to the prosomatic region, _i.e._ to

the region innervated by the trigeminal nerve. An indication of similar

markings is given in Lankester's figure of _Eukeraspis pustuliferus_ (see

Fig. 127, B), and, indeed, evidence of a segmentation under the

antero-lateral border of the head-shield is recognized at the present time,

not only in the Cephalaspidæ, but also in the Pteraspidæ, as was pointed

out to me by Smith Woodward in the specimens at the British Museum. Also,

in _Cyathaspis_, Jaekel has drawn attention to markings of a similar

segmental nature (Fig. 137).

There seems, then, little doubt but that these primitive fishes possessed

something in this region which was of a segmental character, and indicated

at least five segments, probably more.

Rohon entitles his discovery 'the segmentation of the primordial cranium.'

It would, I think, be better to call it the segmentation of {340}the

anterior region of the head, for that is in reality what his figures show,

not the segmentation of the primordial cranium, which, to judge from

Ammocoetes, was confined to the region of the glabellum.

What is the interpretation of this appearance?

[Illustration: FIG. 136.--LATERAL AND DORSAL VIEWS OF THE FRONTAL AND

OCCIPITAL REGIONS OF THE HEAD-SHIELD OF THYESTES, AFTER REMOVAL OF THE

OUTER SURFACE. (From ROHON.)]

[Illustration: FIG. 137.--UNDER SURFACE OF HEAD-SHIELD OF CYATHASPIS. (From

JAEKEL.)

_A._, lateral eyes; _Ep._, median eyes.]

Any segmentation in the head-region must be indicative of segments

belonging to the trigeminal or prosomatic region, or of segments belonging

to the vagus or mesosomatic region. Many palæontologists, looking upon

segmentation as indicative of gills and gill-slits, have attempted to

interpret such markings as branchial segments, regardless of their

position. As the figures show, they extend in front of the eyes and reach

round to the front middle line, a position which is simply impossible for

gills, but points directly to a segmentation connected with the trigeminal

nerve. Comparison with Ammocoetes makes it plain enough that the markings

in question are prosomatic in position, and that the gill-region must be

sought for in the place {341}where Schmidt and Rohon located it in

Thyestes, viz. the so-called occipital region.

This discovery of Rohon's is, in my opinion, of immense importance, for it

indicates that, in these early fishes, the prosomatic segmentation,

associated with the trigeminal nerve, was much more well-marked than in any

fishes living in the present day. Why should it be more well-marked?

Turning to the palæostracan, it is very suggestive to compare the markings

on their prosomatic carapace with these markings. Again and again we find

indications of segmentation in these fossils similar to those seen in the

ancient fishes. Thus in Fig. 138 I have put side by side the palæostracan

_Bunodes_ and the fish _Thyestes_, both life size. In the latter I have

indicated Rohon's segments; in the former the markings usually seen.

From the evidence of Phrynus, Mygale, etc., as already pointed out, such

markings in the palæostracan fossils would indicate the position of the

tergo-coxal muscles of the prosomatic appendages, even though such

appendages have not yet been discovered, and it is significant that in all

these cases there is a distinct indication of a median plate or glabellum

in addition to the segmental markings. Especially instructive is the

evidence of Phrynus, as is seen by a comparison of Figs. 107 and 108, which

shows clearly that this median plate (_glab._) covered the brain-region, a

brain-region which is isolated and protected from the tergo-coxal muscles

by the growth dorsalwards of the flanges of the plastron. In this way an

incipient cranium of a membranous character is formed, which helps to give

attachment to these tergo-coxal muscles. As such cranium is derived

directly from the plastron, it is natural that it should ultimately become

cartilaginous, just as occurs when Ammocoetes becomes Petromyzon and the

cartilaginous cranium of the latter arises from the membranous cranium of

the former. In Galeodes also the growth dorsalwards of the lateral flanges

of the plastron to form an incipient cranium in which the brain lies is

very apparent.

[Illustration: FIG. 138.--A, OUTLINE OF _Thyestes Verrucosus_ WITH ROHON'S

SEGMENTS INDICATED; B, OUTLINE OF _Bunodes Lunula_ WITH LATERAL EYES

INSERTED.

Both figures natural size.]

{342}I venture, then, to suggest that in the Osteostraci the median hard

plate or glabellum protected a brain which was enclosed in a membranous

cranium, very probably not yet complete in the dorsal region--certainly not

complete if the median pineal eyes so universally found in these ancient

fishes were functional--a cranium derived from the basal trabeculæ, in

precisely the same manner as we see it already in its commencement in

Phrynus and other scorpions. With the completion of this cranium and its

conversion into cartilage, and subsequently into bone, an efficient

protection was afforded to the most vital part of the animal, and thus the

hard head-shield of the Palæostraca and of the earliest fishes was

gradually supplanted by the protecting bony cranium of the higher

vertebrates.

Step by step it is easy to follow in the mind's eye the evolution of the

vertebrate cranium, and because it was evolved direct from the plastron,

the impossibility of resolving it into segments is at once manifest; for

although the plastron was probably originally segmented, as Schimkéwitsch

thinks, all sign of such segmentation had in all probability ceased, before

ever the vertebrates first made their appearance on the earth.

This powerful suctorial apparatus is innervated entirely by the trigeminal