TELEOSTEI (_CONTINUED_): ACANTHOPTERYGII-OPISTHOMI-PEDICULATI-
PLECTOGNATHI 650
INDEX 729
{xi}SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK
_The names of extinct groups are printed in italics_.
CHORDATA (p. 3).
I. HEMICHORDATA (p. 3).
Order. Family.
–––––– –––––––
ENTEROPNEUSTA (p. 5)
Glandicipitidae (p. 17).
Ptychoderidae (p. 17).
Harrimaniidae (p. 17).
PTEROBRANCHIA (p. 21)
PHORONIDEA (p. 27)
II. UROCHORDATA = TUNICATA (pp. 4, 35, 63).
Order. Sub–Order. Family. Sub–Family.
–––––– –––––––––– ––––––– –––––––––––
LARVACEA (p. 64)
Kowalevskiidae (p. 68).
Appendiculariidae (p. 68).
ASCIDIACEA (p. 70)
ASCIDIAE SIMPLICES (p. 71).
Clavelinidae (p. 71).
Ascidiidae (p. 72).
Hypobythiinae (p. 72).
Ascidiinae (p. 72).
Corellinae (p. 73).
Cynthiidae (p. 74).
Styelinae (p. 74).
Cynthiinae (p. 75).
Bolteninae (p. 75).
Molgulidae (p. 77).
ASCIDIAE COMPOSITAE(p. 80)
MEROSOMATA (p. 85)
Distomatidae (p. 85).
Coelocormidae (p. 86).
Didemnidae (p. 86).
Diplosomatidae (p. 87).
Polyclinidae (p. 87).
HOLOSOMATA (p. 88)
Botryllidae (p. 88).
Polystyelidae (p. 89).
ASCIDIAE LUCIAE (p. 90)
Pyrosomatidae (p. 91).
THALIACEA (p. 95)
CYCLOMYARIA (p. 95).
Doliolidae (p. 96).
HEMIMYARIA (p. 101).
Salpidae (p. 101).
Octacnemidae (p. 108).
III. CEPHALOCHORDATA (pp. 4, 112).
Branchiostomatidae (p. 137).
IV. CRANIATA (pp. 4, 141).
CLASS––CYCLOSTOMATA (pp. 145, 150, 421).
Sub–Class. Order. Sub–Order. Family. Sub–Family.
–––––––––– –––––– –––––––––– ––––––– –––––––––––
MYXINOIDES (p. 421)
Myxinidae (p. 422).
Bdellostomatidae (p. 423).
PETROMYZONTES (p. 425)
Petromyzontidae (p. 426).
CLASS––PISCES (pp. 145, 431).
ELASMOBRANCHII(p. 431)
_PLEUROPTERYGII_ (p. 436)
_Cladoselachidae_ (p. 438).
_ICHTHYOTOMI_ (p. 440)
_Pleuracanthidae_ (p. 440).
_ACANTHODEI_ (p. 440)
_Diplacanthidae_ (p. 441).
_Acanthodidae_ (p. 441).
PLAGIOSTOMI (p. 442)
SELACHII (p. 442)
Notidanidae (p. 442).
Chlamydoselachidae (p. 443).
Heterodontidae (p. 444).
_Cochliodontidae_ (p. 445).
_Psammodontidae_ (p. 446).
_Petalodontidae_ (p. 446).
Scylliidae (p. 446).
Carchariidae (p. 448).
Sphyrnidae (p. 449).
Lamnidae (p. 450).
Cetorhinidae (p. 453).
Rhinodontidae (p. 454).
Spinacidae (p. 454).
Rhinidae (p. 456).
Pristiophoridae (p. 457).
BATOIDEI (p. 457)
Pristidae (p. 459).
Rhinobatidae (p. 460).
Raiidae (p. 461).
_Tamiobatidae_ (p. 462).
Torpedinidae (p. 462).
Trygonidae (p. 464).
Myliobatidae (p. 465).
HOLOCEPHALI (p. 466)
_Ptyctodontidae_ (p. 468).
_Squaloraiidae_ (p. 468).
_Myriacanthidae_ (p. 468).
Chimaeridae (p. 468).
TELEOSTOMI(p. 475)
CROSSOPTERYGII (p. 476)
_OSTEOLEPIDA_ (p. 437)
_Osteolepidae_ (p. 477).
_Rhizodontidae_ (p. 478).
_Holoptychidae_ (p. 479).
_Coelacanthidae_ (p. 480).
CLADISTIA (p. 481)
Polypteridae (p. 481).
CHONDROSTEI (p. 485)
_Palaeoniscidae_ (p. 486).
_Platysomidae_ (p. 487).
_Belonorhynchidae_ (p. 488).
_Catopteridae_ (p. 488).
_Chondrosteidae_ (p. 489).
Polyodontidae (p. 491).
Acipenseridae (p. 492).
HOLOSTEI (p. 495)
_Semionotidae_ (p. 497).
_Macrosemiidae_ (p. 498).
_Pycnodontidae_ (p. 498).
_Eugnathidae_ (p. 498).
Amiidae (p. 499).
_Pachycormidae_ (p. 501).
_Aspidorhynchidae_ (p. 502).
Lepidosteidae (p. 502).
TELEOSTEI (pp. 504, 541)
MALACOPTERYGII (p. 543)
_Pholidophoridae_ (p. 545).
_Archaeomaenidae_ (p. 545).
_Oligopleuridae_ (p. 545).
_Leptolepididae_ (p. 546).
Elopidae (p. 546).
Albulidae (p. 547).
Mormyridae (p. 549)
Mormyrinae (p. 551).
Gymnarchinae (p. 551)
Hyodontidae (p. 552).
Notopteridae (p. 554).
Osteoglossidae (p. 555).
Pantodontidae (p. 558).
_Ctenothrissidae_ (p. 559).
Phractolaemidae (p. 560).
_Saurodontidae_ (p. 561).
Chirocentridae (p. 561).
Clupeidae (p. 562)
Thrissopatrinae (p. 562).
Engraulinae (p. 563).
Clupeinae (p. 563).
Chaninae (p. 563).
Salmonidae (p. 565).
[_Pachyrhizodontidae_ (p. 569).]
Alepocephalidae (p. 569).
Stomiatidae (p. 570)
Chauliodontinae (p. 571).
Sternoptychinae (p. 571).
Stomiatinae (p. 571).
Gonorhynchidae (p. 572).
Cromeriidae (p. 573).
OSTARIOPHYSI (p. 573)
Characinidae (p. 575)
Erythrininae (p. 575).
Hydrocyoninae (p. 575).
Serrasalmoninae (p. 576).
Ichthyoborinae (p. 576).
Xiphostominae (p. 576).
Anostominae (p. 576).
Hemiodontinae (p. 576).
Distichodontinae
(p. 576).
Citharininae (p. 576).
Gymnotidae (p. 579).
Cyprinidae (p. 581)
Catostominae (p. 581).
Cyprininae (p. 582).
Cobitidinae (p. 582).
Homalopterinae (p. 582).
Siluridae (p. 586)
Clariinae (p. 588).
Silurinae (p. 588).
Bagrinae (p. 588).
Doradinae (p. 588).
Malopterurinae (p. 588).
Callichthyinae (p. 588).
Hypophthalminae (p. 589).
Trichomycterinae
(p. 589).
Loricariidae (p. 594)
Arginae (p. 595).
Loricariinae (p. 595).
Aspredinidae (p. 596).
SYMBRANCHII (p. 597)
Symbranchidae (p. 597).
Amphipnoidae (p. 598).
APODES (p. 599)
Anguillidae (p. 600).
Nemichthyidae (p. 603).
Synaphobranchidae (p. 603).
Saccopharyngidae (p. 603).
Muraenidae (p. 604).
HAPLOMI (p. 605)
Galaxiidae (p. 607).
Haplochitonidae (p. 608).
_Enchodontidae_ (p. 608).
Esocidae (p. 609).
Dalliidae (p. 610).
Scopelidae (p. 611).
Alepidosauridae (p. 614).
Cetomimidae (p. 614).
_Chirothricidae_ (p. 615).
Kneriidae (p. 615).
Cyprinodontidae (p. 616).
Amblyopsidae (p. 618).
Stephanoberycidae (p. 619).
Percopsidae (p. 620).
HETEROMI (p. 621)
_Dercetidae_ (p. 623).
Halosauridae (p. 623).
Lipogenyidae (p. 624).
Notacanthidae (p. 624).
Fierasferidae (p. 625).
CATOSTEOMI (p. 626)
Gastrosteidae (p. 629)
Aulorhynchidae (p. 631)
_Protosyngnathidae_ (p. 631)
Aulostomatidae (p. 632)
Fistulariidae (p. 632)
Centriscidae (p. 633)
Amphisilidae (p. 633)
= Hemibranchii (p. 627).
Solenostomidae (p. 633)
Syngnathidae (p. 634)
= Lophobranchii (p. 628).
Pegasidae (p. 635)
= Hypostomides (p. 628).
PERCESOCES (p. 636)
Scombresocidae (p. 637).
Ammodytidae (p. 639).
Atherinidae (p. 639).
Mugilidae (p. 640).
Polynemidae (p. 640).
Chiasmodontidae (p. 641).
Sphyraenidae (p. 642).
Tetragonuridae (p. 642).
Stromateidae (p. 643).
Icosteidae (p. 644).
Ophiocephalidae (p. 644).
Anabantidae (p. 645).
ANACANTHINI (p. 646)
Macruridae (p. 647).
Gadidae (p. 647).
Muraenolepididae (p. 649).
Sub–Order. Division. Family.
–––––––––– ––––––––– –––––––
ACANTHOPTERYGII (p. 650)
PERCIFORMES (p. 652)
Berycidae (p. 655).
Monocentridae (p. 656).
Pempheridae (p. 656).
Centrarchidae (p. 657).
Cyphosidae (p. 657).
Lobotidae (p. 658).
Toxotidae (p. 658).
Nandidae (p. 658).
Percidae (p. 658).
Acropomatidae (p. 659).
Serranidae (p. 659)
Subfamilies:
* Serraninae (p. 659).
* Grammistinae (p. 660).
* Priacanthinae (p. 660).
* Centropominae (p. 660).
* Pomatominae (p. 660).
* Ambassinae (p. 660).
* Chilodipterinae
(p. 660).
* Lutjaninae (p. 660).
* Cirrhitinae (p. 660).
* Pentacerotinae
(p. 660).
Anomalopidae (p. 660).
Pseudochromididae
(p. 661).
Cepolidae (p. 661).
Hoplognathidae (p. 662).
Sillaginidae (p. 662).
Sciaenidae (p. 663).
Gerridae (p. 663).
Lactariidae (p. 663).
Trichodontidae (p. 663).
Latrididae (p. 663).
Haplodactylidae (p. 664).
Pristipomatidae (p. 664).
Sparidae (p. 664).
Mullidae (p. 665).
Scorpididae (p. 666).
Caproidae (p. 666).
Chaetodontidae (p. 667).
Drepanidae (p. 668).
Acanthuridae (p. 668).
Teuthididae (p. 668).
Osphromenidae (p. 669).
Embiotocidae (p. 670).
Cichlidae (p. 670).
Pomacentridae (p. 672).
Labridae (p. 673).
Scaridae (p. 674).
SCOMBRIFORMES (p. 675)
Carangidae (p. 677).
Rhachicentridae (p. 677).
Scombridae (p. 678).
Trichiuridae (p. 679).
Histiophoridae (p. 679).
_Palaeorhynchidae_
(p. 680).
Xiphiidae (p. 681).
Luvaridae (p. 681).
Coryphaenidae (p. 681).
Bramidae (p. 682).
ZEORHOMBI (p. 682)
Zeidae (p. 683).
_Amphistiidae_ (p. 684).
Pleuronectidae (p. 684).
KURTIFORMES(p. 687)
Kurtidae (p. 687).
GOBIIFORMES(p. 688)
Gobiidae (p. 689).
DISCOCEPHALI(p. 691)
Echeneididae (p. 691).
SCLEROPAREI (p. 692)
Scorpaenidae (p. 694).
Hexagrammidae (p. 696).
Comephoridae (p. 696).
Rhamphocottidae (p. 697).
Cottidae (p. 697).
Cyclopteridae (p. 698).
Platycephalidae (p. 699).
Hoplichthyidae (p. 699).
Agonidae (p. 700).
Triglidae (p. 700).
Dactylopteridae (p. 701).
JUGULARES (p. 702)
Trachinidae (p. 704).
Percophiidae (p. 705).
Leptoscopidae (p. 705).
Nototheniidae (p. 705).
Uranoscopidae (p. 706).
Trichonotidae (p. 706).
Callionymidae (p. 706).
Gobiesocidae (p. 707).
Blenniidae (p. 709).
Batrachidae (p. 710).
Pholididae (p. 711).
Zoarcidae (p. 712).
Congrogadidae (p. 713).
Ophidiidae (p. 713).
Podatelidae (p. 713).
TAENIOSOMI (p. 714)
Trachypteridae (p. 715).
Lophotidae (p. 716).
OPISTHOMI (p. 716)
Mastacembelidae (p. 716).
PEDICULATI (p. 717)
Lophiidae (p. 718).
Ceratiidae (p. 719).
Antennariidae (p. 720).
Gigantactinidae (p. 720).
Malthidae (p. 720).
PLECTOGNATHI (p. 721)
SCLERODERMI (p. 722)
Triacanthidae (p. 722).
Triodontidae (p. 723).
Balistidae (p. 723).
Ostraconiidae (p. 722).
GYMNODONTES (p. 725)
Tetrodontidae (p. 726).
Diodontidae (p. 726).
Molidae (p. 726).
DIPNEUSTI = DIPNOI (p. 505)
_Ctenodontidae_ (p. 505).
_Uronemidae_ (p. 507).
Ceratodontidae (p. 507).
Lepidosirenidae (p. 511).
OF UNCERTAIN POSITION
Order. Family.
–––––– –––––––
_PALAEOSPONDYLIDAE_ (p. 521).
_OSTRACODERMI_ (p. 522) _HETEROSTRACI_ (p. 524) _Coelolepidae_ (p. 524).
_Drepanaspidae_ (p. 525).
_Psammosteidae_ (p. 526).
_Pteraspidae_ (p. 527).
_OSTEOSTRACI_ (p. 527) _Ateleaspidae_ (p. 528).
_Cephalaspidae_ (p. 528).
_Tremataspidae_ (p. 530).
_ANASPIDA_ (p. 531) _Birkeniidae_ (p. 531).
_ANTIARCHI_ (p. 532) _Asterolepidae_ (p. 534).
_ARTHRODIRA_ (p. 535) _Coccosteidae_ (p. 536).
HEMICHORDATA
BY
SIDNEY F. HARMER, Sc.D., F.R.S.
Fellow of King's College, Cambridge.
{3}CHAPTER I
HEMICHORDATA
CHORDATA AND VERTEBRATA—HEMICHORDATA—ENTEROPNEUSTA—EXTERNAL CHARACTERS AND
HABITS—STRUCTURE—GENERA—DEVELOPMENT—PTEROBRANCHIA—_CEPHALODISCUS_ AND
_RHABDOPLEURA_—PHORONIDEA—_PHORONIS_ AND ACTINOTROCHA—AFFINITIES OF THE
HEMICHORDATA.
The Hemichordata, a marine group which includes the worm-like
Balanoglossus, owe much of their interest to the fact that they are
believed by many zoologists to be related to the lower Vertebrates. This
view is one of a number of mutually exclusive hypotheses, which seek to
derive Vertebrate animals from various Invertebrate ancestors. It is
supported by many striking resemblances between Balanoglossus and the
lowest forms which are by common consent regarded as belonging to the
Vertebrate alliance; but it must be distinctly understood that
Balanoglossus is at most the much modified modern representative of extinct
forms which were also the ancestors of Vertebrates.
The axis of the backbone of all Vertebrates is formed by an elastic rod
known as the "notochord" (Figs. 72, 115), which lasts throughout life in
some of the lowest forms, but in the higher forms appears only in the
embryo. The universal occurrence of this structure has been regarded as the
most important characteristic of the Vertebrata and their allies, which are
accordingly grouped together in the Phylum CHORDATA. The members of this
Phylum are further distinguished from other animals by several important
features. Of these one of the most important appears to be the existence of
lateral {4}outgrowths of the pharynx, which unite with the skin of the neck
and form a series of perforations leading to the exterior. These structures
are the gill-slits, and in the Fishes their walls give rise to vascular
folds or gills. With the assumption of a terrestrial life, the higher
Vertebrates lost their gills as functional organs, respiration being then
performed by entirely different organs, the lungs. But even in these cases,
the gill-slits appear in the embryo; and remains of one pair can usually be
recognised in the adult state of even the highest Vertebrates. Another
fundamental characteristic of the Chordata is given by the central nervous
system, which lies entirely above the alimentary canal, just dorsal to the
notochord. Not only does this position of the nerve-centres distinguish the
Chordata from Invertebrates, but a further point of difference is found in
the development. While in Invertebrates the ventral nerve-cord is formed as
a thickening of the ectoderm or outermost layer of the embryo, in the
Chordata the nervous system is usually formed as a longitudinal groove
running medianly along the back of the embryo. This groove closes to form a
tube of nervous matter, the cavity of which always persists throughout life
as the "central canal" of the spinal chord and its anterior prolongation
which constitutes the "ventricles" of the brain.
Although the animals which are considered in this chapter are not admitted
by all zoologists to be related to the Vertebrates, there can be no
question that their respiratory organs closely resemble typical gill-slits.
Since, moreover, they possess structures which can be regarded, with a fair
amount of probability, as agreeing in essential respects with the notochord
and the tubular dorsal nervous system of Vertebrates, it appears
justifiable to include them in the Chordata, which are then subdivided into
(1) HEMICHORDATA, in which a "notochord" occurs in the anterior end of the
body only; (2) UROCHORDATA (Tunicata or Ascidians), in which the notochord
is restricted to the tail; (3) CEPHALOCHORDATA (Amphioxus), in which the
notochord extends the entire length of the body _and of the head_; (4)
CRANIATA, in which a brain is developed as an enlargement of the central
nervous system, the notochord does not extend farther forward than the
middle of the brain, and a vertebral column is present. These last are thus
usually known as Vertebrata, although in distinguishing an "Invertebrate"
from a "Vertebrate" it is more {5}logical to regard all Chordata as
Vertebrates, since the Invertebrata are in no sense a natural group with
common characteristics, their union under one name merely implying that
they have no close affinity to the Vertebrates. It is often convenient in
practice to divide animals into Vertebrates and Invertebrates, but from a
zoological point of view a division of the animal kingdom into Molluscs and
Non-Molluscs would have as much or as little significance.
The sub-phylum Hemichordata[1] consists of the Orders:—(I.)
ENTEROPNEUSTA,[2] including Balanoglossus (Fig. 1); (II.) PTEROBRANCHIA,[3]
represented by the genera _Cephalodiscus_ (Fig. 9) and _Rhabdopleura_ (Fig.
12). To these should possibly be added (III.) PHORONIDEA, for the reception
of _Phoronis_ (Fig. 13).
ORDER I. ENTEROPNEUSTA.
_Worm-like Hemichordata, with numerous gill-slits, a straight intestine,
and a terminal anus. Proboscis separated by a narrow stalk from the simple
ring-shaped collar, which is succeeded by an elongated trunk_.
The structure of Balanoglossus, formerly the sole genus belonging to this
Order, but now divided[4] into the genera _Ptychodera_, _Balanoglossus_,
_Glossobalanus_, _Glandiceps_, _Spengelia_, _Schizocardium_, _Harrimania_,
_Dolichoglossus_, and _Stereobalanus_, has of recent years formed the
subject of elaborate investigations by Spengel,[5] Bateson,[6] and
Willey.[7] More than thirty species are known, ranging in length from 25
mm.[8] (_Pt. bahamensis_) to 2500 mm. (_B. gigas_), and for the most part
inhabiting shallow water; _Glossobalanus sarniensis_ occurring between
tide-marks in the Channel Islands. _Glandiceps talaboti_ has, however, been
dredged near Marseilles from as much as 190 fathoms, while _G. abyssicola_
was found by the "Challenger" at a depth of 2500 fathoms, off the West
Coast of Africa.
{6}[Illustration: FIG. 1.—Forms of Balanoglossus. A, _Balanoglossus
clavigerus_, Eschsch., Naples, × ½; B, _Glandiceps hacksi_, Mar.
(incomplete), Japan, × 1; C, _Schizocardium brasiliense_, Speng., Rio de
Janeiro, × 1; D, _Dolichoglossus kowalevskii_, A.Ag., Chesapeake Bay, ×
1. _a_, Anus; _ab_, abdominal and caudal regions; _b_, branchial region;
_c_, collar; _g_, genital region; _g.p_, gill-pore; _g.w_, genital wing;
_h_, hepatic region; _m_, position of mouth; _p_, proboscis; _t_, trunk.
(A, B, and C from Spengel; D from Bateson.)]
_Balanoglossus_, the largest genus now recognised by Spengel, appears to be
practically world-wide in its distribution; _Schizocardium_ is recorded
from both sides of S. America; _Glandiceps_ from the Atlantic, the
Mediterranean, Japan, and the Indian Ocean; _Spengelia_ from the South
Pacific; and other species from the White Sea to New Zealand. The habitat
is usually sand or gravelly sand, in which the animal forms a kind of tube
by means of the abundant mucus secreted by its skin. _Dolichoglossus
kowalevskii_ (Fig. 1, D), according to Bateson,[9] lives between tide-marks
at a {7}depth of about eight inches. The greater part of the body is coiled
in an even, cork-screw-like spiral, while the anterior end, including the
front part of the branchial region, is maintained in a vertical position.
The posterior end is also kept upright, and can be moved up and down in a
vertical shaft opening on the surface, thus enabling the animal to eject
the undigested sand from its anus.
The coloration of Balanoglossus is often brilliant. That of _D.
kowalevskii_[10] is as follows:—The "proboscis" (cf. Fig. 1, B, _p_) is
yellowish white; the "collar" (_c_) is brilliant red-orange (especially in
males), with a white ring posteriorly; the "trunk" (_t_), the subdivision
of which into "branchial," "genital," "hepatic," "abdominal," and "caudal"
regions is better indicated in other species (Fig. 1, A, _b_, _g_, _h_,
_ab_), is orange-yellow, shading to green-yellow in the semi-transparent
caudal region. The genital region is grey in females and yellow in males, a
sexual difference in colour being common in Enteropneusta. The hepatic
papillae of other species may be bright green.
The odour of _D. kowalevskii_ resembles that of "chloride of lime with a
faecal admixture," while that of _Balanoglossus aurantiacus_ suggests
iodoform. All Enteropneusta are said to have a more or less offensive
smell. A species of Balanoglossus is known to be intensely
phosphorescent.[11]
The mouth (Fig. 7, _m_) is situated on the ventral side, at the base of the
proboscis, and is concealed by the free anterior edge of the collar, which
encircles the thin "proboscis-stalk" (Fig. 3, _p.s_). The animal has the
singular peculiarity of being unable to close its mouth;[12] and thus, as
it burrows through the ground, the sand which passes into the alimentary
canal leaves it in a continuous column through the terminal anus.[13] The
large coiled "castings" formed in this way between tide-marks enable the
experienced collector to infer the presence of Balanoglossus; and in a West
Indian species described by Willey[14] they are so large as to form "an
important feature in the landscape at low tide."
The principal agents in burrowing are the proboscis and collar. An animal
observed by Spengel pushed the tip of its proboscis into the sand, waves of
muscular contraction meanwhile passing {8}over the surface of the
proboscis. At first the animal made slow progress; but the collar, becoming
surrounded by sand, soon became a point of resistance by means of which the
proboscis could bury itself yet more deeply. The animal quickly disappeared
as soon as the first two regions of its body were engaged in the task of
burrowing[15]
This action is due partly to the muscles of the body-wall, but largely to
the power possessed by the proboscis and collar of becoming swollen and
turgid. Spengel has observed that these parts become flaccid when the
animal is taken out of water, and can only swell again when it is replaced
therein; and it may thus fairly be concluded that the enlargement is due to
the taking in of water. This is probably in fact the most important
function of the "proboscis-pore" and of the "collar-pores" which are
described below.
[Illustration: FIG. 2.—Diagram of a dorsal view of a Balanoglossus-embryo,
after the formation of the body-cavities, _a_, Alimentary canal; _b.c_^1,
body-cavity of the proboscis; _b.c_^2, of the collar; _b.c_^3, of the
trunk. (From Bateson.)]
BODY-CAVITIES.—The existence of five separate body-cavities (Fig. 2) is one
of the most fundamental facts in the anatomy of Balanoglossus. The first
body-cavity, or cavity of the proboscis (_b.c_^1), is single and unpaired;
the second body-cavities (_b.c_^2) are paired spaces, one belonging to each
side of the collar; the third body-cavities (_b.c_^3) are similarly paired,
and correspond with the trunk. While there is no connection between
successive body-cavities, there are in certain regions communications
between the two cavities of the same pair. Each of the paired cavities is
at one time a closed lateral space between the skin and the alimentary
canal. As the two spaces which constitute the pair grow towards one
another, both above and below the alimentary canal, they come into such
close apposition that they remain separated only by their conjoined walls.
In this way are formed the dorsal and ventral mesenteries (Fig. 4, _d.m_,
_v_), the former being the only one to persist in the higher Vertebrates.
The body-cavities of the adult become to a large extent disguised by being
traversed by connective tissue and muscles.
{9}The hinder part of the proboscis-cavity is divided by the forward growth
of the notochord (Fig. 3, _n_) into dorsal and ventral portions. The dorsal
cavity in extending backwards becomes further subdivided into right and
left halves, the latter typically opening dorsally to the exterior on the
proboscis-stalk by an asymmetrical "proboscis-pore" (_p.p._). Two
symmetrical proboscis-pores may, however, occur, or a median pore connected
with the left division of the proboscis-cavity. These may be individual
variations within the limits of a single species, or may occur as a normal
feature of a species.
[Illustration: FIG. 3.—Dorsal view of the anterior end of the body of
_Dolichoglossus kowalevskii_, × 3. _c_, Collar; _c.n_, circular nerve;
_c.p_, collar-pore; _d_, dorsal nerve; _g_, gill-pore; _n_, notochord;
_n.s_, central nervous system, showing the anterior and posterior
neuropores; _p_, proboscis; _p.p_, proboscis-pore; _p.s_, proboscis-stalk;
_t_, trunk; _v_, ventral nerve. The nerve-plexus of the proboscis is
represented as a black line. (After Bateson.)]
The collar-cavities open by two "collar-pores" (Fig. 3, _c.p._), situated
at the posterior end of the collar, into the first pair of gill-pouches,
near their external opening. Willey has recently described[16] vestigial
pores in relation with the "perihaemal spaces," a pair of dorsally situated
outgrowths of the third body-cavities into the collar-region. Narrow
"peripharyngeal spaces," also a forward growth of the third body-cavities,
closely invest the pharynx in some species.
BODY-WALL AND NERVOUS SYSTEM.—The body-wall (Fig. 4) consists externally of
a thick ciliated epidermis (_e_), containing numerous gland-cells which
secrete an abundant mucus. Beneath the epidermis is a basement-membrane,
while more internally are layers of muscles, whose arrangement differs in
different parts of the body and in different species.
The nervous system consists of a plexus of cells and fibres which lie in
the basal part of the epidermis of all parts of the animal, outside the
basement-membrane; the thicker portions of the plexus forming definite
nerve-tracts. This intimate connexion {10}between the epidermis and the
nervous system is usually restricted to embryonic life in other animals.
[Illustration: FIG. 4.—_Ptychodera bahamensis_, Bahama Is. Transverse
section through the branchial region. _b_, Branchial part of pharynx;
_b.c_^3, third body-cavity; _d.m_, dorsal mesentery; _d.n_, dorsal nerve;
_d.v_, dorsal vessel; _e_, epidermis, with nerve-layer (black) at its base;
_g_, genital wing; _g.p_, gill-pore, encroached on by the tongue-bar (_t_);
_l_, lateral septum; _m_, longitudinal muscles; _o_, oesophageal or
alimentary part of pharynx; _r_, reproductive organ; _t_, tongue-bar; _v_,
ventral mesentery and ventral vessel; _v.n_, ventral nerve. (After
Spengel.)]
The main nerves of Balanoglossus are a dorsal and a ventral tract in the
trunk region (Fig. 4, _d.n_, _v.n_), a circular tract (Fig. 3, _c.n_)
connecting these two at the posterior edge of the collar, and a strong
concentration of nerve-tissue round the whole of the proboscis-stalk, and
of the posterior end of the proboscis (Fig. 3). In the region of the collar
the nervous system attains its highest development, taking the form of a
median cord passing above the alimentary canal. This cord, known as the
central nervous system (Fig. 7, _n.s_), runs through the cavity of the
collar, but is connected with the epidermis at each end. It thus becomes
continuous in front with the nerve-layer on the proboscis-stalk, while
posteriorly it passes into the dorsal and the circular nerve-tracts. In
nearly all cases the epidermis is pushed into the cord at the points where
it passes into the skin, in the form of an anterior and a posterior
"neuropore" (Fig. 3). A transverse section through the extreme front or
hind end of the collar accordingly shows a tubular nervous system. In
certain species, as in _Glossobalanus sarniensis_ and _Ptychodera flava_, a
central canal, opening in front and behind, exists throughout the entire
length of the central nervous system, while in _G. minutus_ a canal of this
kind occurs in the young animal, but not in the adult. The central nervous
system is developed as a longitudinal {11}dorsal groove in the larva,[17]
and in a similar manner in the collar which is formed as the result of
regeneration after injury.[18] Balanoglossus is thus typically provided
with a dorsal, tubular, central nervous system, and although this
arrangement does not extend beyond the limits of the collar, it shows a
noteworthy resemblance to Vertebrate animals.
In some cases the central nervous system is connected with the dorsal
epidermis by a varying number (1-17) of median "roots," which have been
compared by Bateson with the dorsal roots of the spinal nerves of
Amphioxus, and are probably remains of the embryonic connexion of the
collar nervous system with the dorsal epidermis.
ALIMENTARY CANAL.—The mouth (Fig. 7, _m_) leads widely into the alimentary
canal, which, passing through the collar, enters the branchial region,
where it is characterised by the existence of communications with the
exterior. These, the gill-slits, are developed, as in Vertebrates, as
paired outgrowths of the alimentary canal, and new gill-slits are
constantly being formed at the posterior end of the branchial region with
advancing age. The maximum number of the gill-slits, and the extent of the
branchial region, are by no means uniform throughout the Enteropneusta.
Thus _Dolichoglossus otagoensis_ is said to have no more than 12 pairs,
_Glossobalanus minutus_ only 40 pairs, while _Balanoglossus aurantiacus_
may have as many as 700 pairs. In _Ptychodera flava_ the variation is so
great that Willey distinguishes[19] two extreme conditions as
"macrobranchiate" and "brachybranchiate" respectively, although
intermediate conditions are also found. It should be noted that
Balanoglossus agrees with Amphioxus in the indefinite number of the
gill-slits.
The gill-slits usually have the form of the so-called "branchial pouches"
or "gill-sacs" (Figs. 5, 6, _g.s_). Each ordinarily opens to the exterior
by a small pore (Fig. 1, D, 5, _g.p_) or slit, situated on the dorsal side,
in a shallow longitudinal groove not far from the middle line. The gill-sac
has a complete wall of its own, and lies between the alimentary canal and
the body-wall, communicating with the former by a U-shaped slit. While a
dorsal {12}view of the animal thus shows a linear series of simple pores, a
view of the pharynx from the inside appears as in Fig. 5.
At the hind end of the pharynx the inner opening of the developing gill-sac
is circular. Slightly further forward the dorsal side of the pore is
indented into a crescent, which grows longer in a dorso-ventral direction,
and becomes a U, whose two limbs are nearly separated by a mass of tissue,
the so-called "tongue-bar" (Fig. 5, _t_). The special interest of this mode
of development is that it is identical with what occurs in Amphioxus (p.
120), which is universally admitted to belong to the Chordata.
The gill-sacs of Balanoglossus follow one another closely, the hind wall of
one being in contact with the front wall of the next, and constituting a
"branchial septum" (_b.s_). Both septa and tongue-bars are supported by
chitinous rods, which are special thickenings of the membrane at the base
of their epithelium. Two rods occur in each tongue-bar, separated by an
interval of body-cavity (Figs. 5, 6), and only one rod in each septum.
Originally of this form—∩∩ ∩∩—the rods have joined in pairs, the united
limbs forming the single rod of each branchial septum. In this respect
again we have a similarity between Balanoglossus and Amphioxus, except that
in the latter the concrescence proceeds one step farther, and the two rods
of the tongue-bar unite, like those of the branchial septum. The latter,
the so-called "primary" skeletal rods of Amphioxus, are forked ventrally as
in Balanoglossus (Fig. 5).
[Illustration: FIG. 5.—Diagram of two gill-sacs of Balanoglossus, seen from
the inside of the pharynx. _b_, Branchial skeleton, consisting of a single
forked bar in each branchial septum (_b.s_), and of two bars in each
tongue-bar; _g.p_, gill-pore, opening on the dorsal surface of the trunk;
_g.s_, gill-sac; _s_, synapticulum (only one or two shown); _t_,
tongue-bar. The arrows indicate the communications of the gill-sacs with
the exterior and with the pharynx.]
In Amphioxus, as in most Enteropneusta, adjacent rods are {13}connected at
intervals by chitinous "synapticula" (Fig. 5, _s_), which traverse one or
the other of the halves of the gill-slit. In _Dolichoglossus_, where no
synapticula occur, the tongue-bars may be turned inside out by slight
pressure, and then project to the exterior through the gill-pores.
The subdivision of the branchial region of the alimentary canal into two
parts, as shown in Fig. 4, is characteristic of _Glossobalanus_ and its
allies. In _Dolichoglossus_ and _Glandiceps_ there is no such constriction,
the region occupied by the gill-slits being merely the dorsal half of a
tube with a simple circular section. _Schizocardium_ (Fig. 6) agrees with
Amphioxus in the fact that the gill-slits occupy nearly the whole of the
wall of the pharynx; the only parts not perforated by gill-slits being the
small dorsal and ventral portions.
In _Ptychodera_ (Fig. 4), the gill-sacs are practically absent. The
U-shaped slits of the pharyngeal wall thus open directly to the
exterior,[20] and can be seen from the outside. In species which have this
arrangement, the genital wings are greatly developed, so as to arch over
the back of the branchial region. The gill-slits thus open into a kind of
"atrium," resembling that of Amphioxus in its relation to the gill-slits,
and in having the generative organs on its outer side, but differing from
it in being dorsal to the pharynx.
[Illustration: FIG. 6.—_Schizocardium brasiliense_; transverse section
through the branchial region, showing the great extent of the branchial
part (_b_) of the pharynx; the oesophageal part (_o_) is reduced to a mere
groove; _g_, gill-pore; _g.s_, gill-sac; _r_, reproductive organ; _s_,
synapticula (cf. Fig. 5); _t_, tongue-bar. The muscles of the body-wall are
not indicated: in other respects the figure corresponds with Fig. 4, except
for the absence of genital wings in this region of the body. (After
Spengel.)]
At a certain distance behind the branchial region, the alimentary canal in
_Balanoglossus_ and _Schizocardium_ is produced into a series of
outgrowths, into which food does not pass. These "liver-sacs" give rise to
corresponding folds (Fig. 1, A, _h_) {14}of the dorsal body-wall, a
conspicuous external feature of the species in which they are present. The
most interesting peculiarity of the digestive tract in this region is the
existence, in certain species, of pores, possibly vestigial gill-slits,
leading from it to the exterior.
NOTOCHORD AND SKELETON.—The structure compared by Bateson with the
Vertebrate notochord is a hollow dorsal outgrowth of the alimentary canal
of the collar-region (Fig. 7, _n_). Near its origin it is slender, but in
the proboscis it dilates into a comparatively large organ, which in most
cases retains its cavity. Its cells have a vacuolated appearance, which
recalls the fine structure of the Vertebrate notochord. In _Schizocardium_
and _Glandiceps_, the organ is produced into a slender "vermiform process"
(_v_), which extends nearly to the tip of the proboscis.
[Illustration: FIG. 7.—_Schizocardium brasiliense_; longitudinal, median
section through the proboscis, the collar, and the first part of the trunk;
_b_, main blood-space of the proboscis; _b.c_^1, _b.c_^2, _b.c_^3, first,
second and third body-cavities; _c.m_, circular muscles of proboscis; _e_,
epidermis; _l.m_, longitudinal muscles of proboscis; _m_, mouth; _n_,
notochord; _n.s_, central nervous system, continuous with the subepidermic
nerve-plexus (black) of the proboscis, and with the dorsal nerve (_d_);
_p.c_, pericardium; _p.s_, proboscis-stalk; _s_, proboscis-skeleton; _v_,
vermiform process of notochord. (After Spengel.)]
The main support of the proboscis-stalk is the "proboscis-skeleton" (_s_),
a Y-shaped organ whose median part lies beneath the base of the notochord,
its diverging legs extending backwards along the outer side of the
alimentary canal of the collar. The proboscis-skeleton, like the branchial
skeleton, is a special development of the structureless membrane which is
found at the base of the layers of cells {15}of Balanoglossus, and in most
species it grows merely by the deposition of laminae of chitin from the
notochord, and from the ventral epidermis of the proboscis-stalk.
In some species, however, and particularly in _Balanoglossus aurantiacus_
and _Glandiceps_, the primary skeleton becomes surrounded by an extensive
development of a secondary cartilaginoid skeleton, consisting of a
structureless substance into which the adjacent body-cavities of the
proboscis and collar send cellular outgrowths. The possibility of a
relation between this tissue, more or less surrounding a part of the
notochord, and the cartilage of Vertebrates cannot be overlooked.
The caudal region may be stiffened (?) by a "pygochord"[21] which is a
median derivative of the alimentary canal on its ventral side.
VASCULAR SYSTEM AND PROBOSCIS-GLAND.—The main vessels are a dorsal and a
ventral vessel (Fig. 4, _d.v_, _v_), lying in their respective mesenteries.
The details of the vascular system are complicated, and have not been
thoroughly made out, the nearly colourless character of the blood making
their investigation a difficult matter. The following points may, however,
be noted. The blood is said to pass forwards in the dorsal vessel, which,
like the ventral vessel and a pair of lateral vessels in the hepatic
region, is contractile. In the collar the dorsal vessel lies between the
two perihaemal spaces, on the dorsal side of the base of the notochord. The
principal blood-space in the proboscis (Fig. 7, _b_) lies between the
notochord (_n_) and an organ known as the "heart-vesicle" or "pericardium"
(_p.c_). The latter has muscular walls and it contracts rhythmically in the
larva. Its behaviour in the adult is not so easily made out, but it is
probable that, although it does not communicate with the vascular system,
its contractions propel the blood contained in the space immediately
beneath it. The blood, after passing to a glandular organ, the
"proboscis-gland" or "glomerulus," which lies at the sides and in front of
the notochord, appears to pass round the collar to the ventral vessel.
Various systems of vessels are connected with the skin, the gills, the
alimentary canal and the generative organs.
The function of the proboscis-gland is possibly excretory. In this case it
is probable that the proboscis-pore eliminates the {16}waste products
discharged by the gland into the anterior body-cavity, though this view is
not favoured by Willey.
REPRODUCTIVE ORGANS.—The sexes are separate, the reproductive organs
consisting of a series of simple or branched glands which occur along the
dorso-lateral lines of the anterior part of the body; being usually found
throughout the branchial and generative regions and ending at the beginning
of the hepatic region. The reproductive organs may pass into great
extensions of the body-wall known as the "genital wings," specially
developed in some species of _Balanoglossus_ and _Ptychodera_ (Figs. 1 A,
4).
_Stereobalanus canadensis_, a species with long slit-like external
gill-pores, is interesting in possessing a well-developed genital wing both
dorsally and ventrally to the series of gill-pores of each side.
Each reproductive gland opens by its own pore or pores directly to the
exterior. Several glands and pores may occur in the same transverse
section.
According to Spengel there is no definite relation between the number of
the reproductive organs and that of either the gill-sacs or the
liver-outgrowths. The only definite segmentation exhibited by Balanoglossus
is thus the division into three regions which is so distinctly shown by the
arrangement of the body-cavities; though the gill-sacs may indicate an
incipient further segmentation of the major part of the body. In this
connexion it is interesting to notice MacBride's statement[22] that the
body-cavity of Amphioxus develops in the embryo as five cavities, just as
in Balanoglossus; the segmented part of the body being formed by a
secondary segmentation of the third body-cavities.
REGENERATION.—Balanoglossus, like _Phoronis_ (p. 30), possesses great
powers of regenerating lost parts. The posterior part of the body is
readily re-formed, while Spengel has shown[23] that even the proboscis,
collar and branchial region can be regenerated, apparently from a fragment
of the body.
GENERA OF ENTEROPNEUSTA.—Spengel, whose Monograph is indispensable to every
student of the Enteropneusta, formerly {17}proposed to divide the old genus
_Balanoglossus_ into four; but he now recognises no less than nine.[24]
Some of the more important characters are given below, but for the
arrangement of the muscles, important from a systematic point of view,
reference must be made to the original sources.
A. Notochord with a vermiform process (Fig. 7, _v_); pericardium
with anterior diverticula more or less developed. GLANDICIPITIDAE
(_a_) Liver-sacs and synapticula present; gill-slits almost equalling
the pharynx in depth, so that the ventral, non-branchial part of
the pharynx is reduced to a mere groove (Fig. 6); nerve-roots
absent; pericardial diverticula long. _Schizocardium_, Speng.
(_b_) Liver-sacs absent;[25] ventral part of pharynx well developed;
pericardial diverticula short.
(i.) Synapticula and nerve-roots absent. _Glandiceps_, Speng.
(ii.) Synapticula present; nerve-roots present or absent;
genital region with dermal pits. _Spengelia_, Willey.
B. Notochord with no vermiform process; pericardium simple; ventral
part of pharynx large, and sometimes more or less separated from
the branchial part (Fig. 4).
(_a_) Liver-sacs,[26] synapticula and nerve-roots present.
PTYCHODERIDAE
(i.) Genital wings well developed.
(α) Gill-sacs opening by long slits. _Ptychodera_, Eschsch.
(β) Gill-sacs opening by small pores.
_Balanoglossus_, Delle Chiaje.
(ii.) Genital wings hardly developed. _Glossobalanus_, Speng.
(_b_) Liver-sacs, synapticula and nerve-roots absent. HARRIMANIIDAE
(i.) Proboscis long; one proboscis-pore. _Dolichoglossus_, Speng.
(ii.) Proboscis short; two proboscis-pores.
(α) Two pairs of genital wings. _Stereobalanus canadensis_, Speng.
(β) No genital wings. _Harrimania_, Ritter.
The name _Balanoglossus_ was introduced by Delle Chiaje in 1829 for _B.
clavigerus_ (Fig. 1, A), from the neighbourhood of Naples. As Spengel has
shown, its etymology has been much misunderstood. The second half of the
name refers to a fancied resemblance between the Balanoglossus, with its
largely developed genital wings, and the tongue of an ox. Βάλανος means
"acorn," and it has usually been supposed that this name was suggested by
the resemblance of the proboscis, projecting from the collar, to an acorn
in its cup, a view which finds its expression in the {18}name "Eichelwurm"
used by German zoologists. But the idea expressed by Delle Chiaje was
really a similarity between the collar of _Balanoglossus_ and the outer
shell of _Balanus_, the barnacle or "acorn-shell" found everywhere on rocks
between tide-marks.
[Illustration: FIG. 8.—Metamorphosis of Balanoglossus, probably of
_Balanoglossus biminiensis_ Willey, Bahama Islands. All the figures are
magnified to the same scale (× 14). A, fully developed free-swimming larva,
or _Tornaria_, side view; B, commencement of metamorphosis, side view; C,
later stage, dorsal view. Increase in size takes place after this stage;
_a_, anus; _b.c_^1, body-cavity of proboscis; _c_, collar; _c.r_,
transverse ciliated ring; _d.p_ (in A), dorsal pore (= proboscis-pore),
seen also in C on the left side, just behind the reference line _p.c_; _e_,
eyes and sensory thickening of skin (in A); _g_, gill-pore; _g.s_,
gill-sacs, developing as outgrowths of the alimentary canal; three are
already present in B, but are better seen in C, in which they are still
without openings to the exterior; _l_, postoral part of the longitudinal
band of cilia; _l_′, its praeoral part; both _l_ and _l_′ are produced (in
A) into tentacles, over which the band of cilia is looped; the groove in
the middle of the figure, between _l_ and _l_′, conducts the food by the
transverse groove to the mouth (_m_); _p.c_, blood-space of proboscis and
pericardium ("heart" of larva); _s_, stomach. (After Morgan).]
DEVELOPMENT.—The free-swimming, larval stage of Balanoglossus is known as
_Tornaria_ (Fig. 8, A). Several distinct forms of the larva are known,[27]
although it is not yet possible to refer them with certainty to their
respective adults.
{19}Tornaria was described and named by Johannes Müller, who regarded it as
the larva of a Starfish,[28] in spite of his intimate knowledge of the
development of these animals. Its correct systematic position was first
demonstrated by Metschnikoff in 1869.
The larva agrees with many other pelagic forms in being excessively
transparent. The form described by Spengel as _T. grenacheri_ attains the
remarkable length of 9 mm. (nearly ⅖th inch).
The full-grown larva is usually ovoid, and a complicated "longitudinal"
band of cilia runs in several loops over its anterior two-thirds. In side
view, part of the surface limited by the ciliated band appears like a T
with a double outline, the cross piece being bent downwards on each side,
so as to form an anchor-like curve, the middle of which is at the anterior
pole of the larva. In _T. krohni_, which occurs on our south coast,[29] the
ciliated band has a wavy course. In the West Indian larva[30] shown in Fig.
8 A, the ciliated band is produced into numerous tentacles, which fringe
the sides of the T-shaped areas or grooves of the surface. These grooves
and the cilia which border them are used for conveying food to the
mouth.[31] At the apex of the larva is a thickening (_e_) of the ectoderm,
bearing two eye-spots. The main locomotor organ is a simple transverse band
(_c.r_) of "membranellae," vibratile structures composed of fused cilia.
The mouth (_m_), on the ventral side, leads into the oesophagus, and this
into the stomach (_s_). The latter is separated by a marked constriction
from the intestine, which opens by the anus (_a_) at the posterior pole.
On the dorsal side is a pore, the "dorsal pore" (_d.p._), which leads into
a thin-walled sac (_b.c_^1) destined to become the proboscis-cavity of the
adult. To the right of the dorsal pore lies the pulsating "heart," which
apparently becomes the pericardium of the adult. Bourne and Spengel regard
it as a right proboscis-cavity. In the older larvae, the second and third
body-cavities appear as paired thin-walled sacs in close contact with the
hinder part of the stomach. The skin is very thin, and the five
body-cavities do not nearly fill the space between it and the
{20}alimentary canal. This space becomes obliterated for the most part by
the enlargement of the body-cavities, and its last remains persist, as in
many other animals,[32] as the vascular spaces of the adult.
In _Dolichoglossus kowalevskii_, and probably in other species with large
eggs,[33] development proceeds by gradual stages to the adult form, and no
Tornaria-stage is passed through. The opaque young animal, on being
hatched, creeps about in the muddy sand in which the adult is found, later
moving in a leech-like manner, by alternately attaching itself by its two
ends. The young stages were ingeniously obtained by Bateson, to whom our
knowledge of the development of this species is due,[34] by allowing a
large quantity of the mud to settle after being stirred up, the layer of
the specific gravity corresponding with that of the young Balanoglossus
being then separated by means of a siphon. The young stages previously
contained in several hundredweight of mud were thus easily collected into a
pint of water. Morgan recommends treating the layer obtained by a similar
process with picric acid, which stains the young Balanoglossus yellow.
The embryo early becomes a "blastosphere" or hollow vesicle formed of a
single layer of cells. One half of this is invaginated, or pushed into the
other half, and a "gastrula" is thus developed, the cavity of which is the
"archenteron," and the two cell-layers respectively "ectoderm" and
"endoderm." The "blastopore," or orifice of invagination, is at the
posterior pole of the larva, where it narrows and closes, the locomotor,
transverse band of cilia developing round it. No other bands of cilia
appear in this form of development. The proboscis becomes marked out
externally by the appearance of a circular groove, near the middle; and
behind this groove a second one appears, which forms the posterior boundary
of the collar. The larva, which now resembles Fig. 8 C, is usually hatched
at this stage. Two gill-slits make their appearance, and the mouth and anus
are perforated; the anus being in the position of the blastopore.
{21}The body-cavities are formed as five derivatives of the archenteron.
One of these is unpaired, and becomes the proboscis-cavity; while the
others are the paired cavities of the collar and trunk (cf. Fig. 2). There
is some uncertainty about the origin of the body-cavities of the
free-swimming Tornaria, although it seems most probable that they are
developed either from the wall of the stomach or intestine,[35] or from
scattered mesoderm cells[36] which lie in the segmentation-cavity.
The metamorphosis of Tornaria is accompanied by a great diminution in
size,[37] probably due to the loss of water; by this cause and by the
simultaneous thickening of the skin, the larva loses its transparency.
The external features of the metamorphosis are sufficiently indicated by
Fig. 8, the ciliated bands finally disappearing. The dorsal pore persists
as the proboscis-pore; the notochord and numerous gill-slits are developed
as outgrowths of the alimentary canal, the reproductive organs make their
appearance, probably from the mesoderm,[38] the trunk meanwhile elongating
so that the proportions of the adult are acquired.
ORDER II. PTEROBRANCHIA.
_Tubicolous Hemichordata, with one pair of gill-slits or none, a_
U-_shaped alimentary canal, and a dorsal anus situated near the mouth.
Proboscis flattened ventrally into a large "buccal disc," its base
covered dorsally by the collar, which is produced into two or more
tentaculiferous arms. Trunk short, prolonged into a stalk. Reproduction
by budding occurs_.
This group consists of the two genera _Cephalodiscus_ (Fig. 9) and
_Rhabdopleura_ (Fig 12). The latter, first dredged by G.O. Sars, in 1866,
from 120 fathoms off the Lofoten Islands, was included in a catalogue of
deep-sea animals published by his father, M. Sars, in 1868 as _Halilophus
mirabilis_, a name which has been superseded by _Rhabdopleura normani_,
Allman,[39] based on specimens dredged by Canon Norman in 90 fathoms, off
the Shetland Islands.
{22}[Illustration: FIG. 9.—_Cephalodiscus dodecalophus_, M‘Intosh, Straits
of Magellan; A, small portion of the common "house," × 1; _a_, a single
individual, shown also as B, × 65; six of the tentacular arms, belonging to
the collar, are seen springing from behind the proboscis or "buccal disc."
This has a crescentic band of pigment parallel with its posterior border,
which conceals the mouth. The stalk, bearing a bud, which already shows the
beginning of two tentacular arms, is seen to the right. (After M‘Intosh, B
from Parker and Haswell.)]
{23}The structure of _Rhabdopleura_ has been described by Sars,[40]
Lankester,[41] and Fowler.[42] _R. normani_ is common in certain Norwegian
Fjords, at depths of 40 fathoms or more, and has been recorded by Fowler
from the Tristan d'Acunha group in the S. Atlantic; _R. compacta_ has been
found off the N.E. coast of Ireland[43] and near Roscoff, on the N. coast
of Brittany; while forms described by Jullien[44] as _R. grimaldii_ and _R.
manubialis_ have been dredged off the Azores. I have recently found a
fragment of _Rhabdopleura_ from South Australia. It is doubtful how far
these species are distinct.
_Cephalodiscus dodecalophus_[45] was found in the Straits of Magellan,
during the "Challenger" voyage, at a depth of 245 fathoms, and has recently
been rediscovered in shallower water in the same neighbourhood by the
Swedish Antarctic Expedition. Another _Cephalodiscus_, at present
undescribed, has been obtained by Dr. Levinsen from 100 fathoms off the
coast of Japan; while the Dutch expedition carried out by the "Siboga" has
resulted in the discovery of two other specimens, one from a reef close to
low-tide mark on the coast of Borneo, the other from 41-52 fathoms off
Celebes. These three specimens differ markedly from one another and from
the "Challenger" specimen of _C. dodecalophus_, and it is probable that
they all belong to new species. The occurrence of a deep-sea animal at a
great distance from the locality at which it was first found is not in
itself a matter for great surprise; but in the present instance two of the
newly discovered forms are from shallow water, and one of them is actually
littoral. The occurrence of so many species of _Cephalodiscus_ in Oriental
waters suggests that the Pacific or the Indian Ocean may be the
headquarters of the genus, which may prove to be far less of a rarity than
has hitherto been {24}supposed. There is evidence derived from the results
of the "Siboga" expedition that abyssal animals may migrate into
comparatively shallow water in the Malay Archipelago.
_Cephalodiscus_ and _Rhabdopleura_ are remarkable for their power of
producing buds. In the former these arise from the apex of a stalk which is
given off on the ventral side of the body, and they break off when they
reach a certain age; in the latter they do not become free, and a colony
results, which consists of a creeping "stolon" from which vertical branches
are given off at intervals, each ending in an individual of the colony.
_Cephalodiscus_ forms a gelatinous "house" (Fig. 9, A), in the passages of
which are found large numbers of the free individuals, together with their
eggs and embryos. _Rhabdopleura_ (Fig. 12) is protected by cylindrical
tubes, one of which corresponds with each individual.
[Illustration: FIG. 10.—Longitudinal median section of _Cephalodiscus
dodecalophus_. _a_, Anus; _b.c_^1, _b.c_^2, _b.c_^3, first, second, and
third body-cavities; _int_, intestine; _m_, mouth; _nch_, notochord; _n.s_,
central nervous system; _oes_, oesophagus; _op_, operculum, the
ventro-lateral part of the collar; _ov_, ovary; _ovd_, pigmented oviduct;
_ph_, pharynx; _p.p_, proboscis-pore; _ps_, proboscis; _st_, stomach;
_stk_, stalk.]
_Cephalodiscus_, though no more than two or three millimetres in length, is
provided with practically all the important organs possessed by
Balanoglossus. Its proboscis or "buccal shield" (Fig. 10, _ps_) is a large
flattened structure, which overhangs and entirely conceals the mouth. The
anterior body-cavity opens to the exterior by two symmetrically placed
proboscis-pores (_p.p_), just in front of the tip of the notochord (_nch_).
The collar, which has paired body-cavities, is produced dorsally into 4-6
{25}pairs of plume-like arms, which bear an immense number of
pinnately-arranged tentacles. The arms, which may end in a swollen
bulb,[46] have ventral grooves along which food doubtless travels to the
mouth by ciliary currents. The anterior edge of the ventral half of the
collar is drawn out into a narrow flap or operculum (Fig. 11, _op_), in
front of which is the mouth, and behind it the gill-slits (_g_) and
collar-pores (_c_). The central nervous system (_n.s_) is a thick mass of
nerve-tissue in the dorsal epidermis of the collar; it is not sunk beneath
the skin as in Balanoglossus. The details of the nervous and vascular
systems, and the development of the buds, have been described by Masterman.
In the dorsal region of the collar the alimentary canal has a slender
diverticulum, the notochord, which passes into the base of the proboscis;
it is believed by Masterman to have a function similar to that of the
neural gland (cf. p. 52) of Tunicates.
The next part of the alimentary canal, the pharynx,[47] has a single pair
of simple gill-slits opening to the exterior immediately behind the
collar-pores. The short oesophagus (Fig. 10, _oes_) is followed by the wide
stomach (_st_), and this by the intestine (_int_), which opens by the anus
(_a_) near the front end of the body.
[Illustration: FIG. 11.—Longitudinal section through _Cephalodiscus
dodecalophus_, passing through the two sides of the body; _a_, tentacular
arm; _b.c^2_, collar-cavity; _b.c^3_, trunk-cavity; _c_, collar-pore; _g_,
gill-slit; _i_, intestine; _n.s_, central nervous system; _o_, oesophagus;
_op_, operculum; _p_, pharynx; _s_, stomach.]
The trunk contains paired third body-cavities (_b.c^3_), the septum between
which and the collar-cavities is slightly behind the line of origin of the
operculum. Two ovaries (_ov_) are situated between the pharynx and the last
part of the intestine, each opening to the exterior dorsally between the
central nervous {26}system and the anus. Each oviduct (_ovd_) contains dark
pigment, which is seen through the dorsal skin on removing the tentacular
arms. Eggs, each enclosed in a stalked membrane, occur in numbers in the
cavities of the gelatinous house. The early stages of the development are
passed through inside the tubes; but there is at present little other
information with regard to the embryonic development of the Pterobranchia.
The specimen obtained by the "Siboga" from Celebes is a male colony with
dimorphic individuals, the reproductive organs being confined to two-armed
zooids with vestigial alimentary canal.
[Illustration: FIG. 12.—Small portion of colony of _Rhabdopleura normani_,
Allman, Lofoten Islands, × 16. _a_, Anus; _p_, proboscis (= buccal disc);
_r_, rod-like axis of the adherent part of the colony, prolonged into _s_,
the stalks of the individuals; _st_, stomach; _t_, the two tentacular arms
of the collar. (After Sars.)]
_Rhabdopleura_ differs from _Cephalodiscus_ in its much smaller size,[48]
and it is perhaps due to its minuteness that it does not possess certain
organs found in the latter. The stalk is represented by a long muscular
cord, which is merely a narrow part of the body. Basally the stalk of each
individual passes into a common axis, which is for the most part attached
to the substance on which the colony is growing, and is to some extent
branched. The muscular stalk can be contracted into a spiral, thereby
retracting the animal into its tube. The stalks and the younger parts of
the axis which connects them are soft, but the older parts secrete a dark
brown cuticle, forming a narrow {27}tube which becomes embedded in the
adherent wall of the outer tube. The thin dark axis, to which the name
_Rhabdopleura_ refers, is the feature by which the animal can most readily
be recognised without magnification.
The outer transparent tube is constructed by the proboscis, or buccal
shield, the secretion of which appears to be intermittent, so that the tube
consists of a series of rings piled on one another. The animal crawls up
the inside of its tube by means of its proboscis, while it is retracted by
means of the muscles of its stalk.
The growing axis ends in a row of young buds, the buccal shields of which
early reach a relatively large size. The terminal bud gives rise to
tube-rings, so that the axis is surrounded by a cylindrical outer tube,
which becomes interrupted by transverse septa, each bud, except the end
one, thus lying in a closed chamber. The wall of each chamber becomes
perforated, and the buccal shield then prolongs this perforation by adding
tube-rings, the formation of which continues till the tube reaches a
considerable length. The bud remains connected with the axis by means of
its narrow proximal region, which forms its stalk. The adherent part of the
adult colony thus consists of a row of short tubes, traversed by the common
axis of the colony. Each tube is produced laterally into the upright tube
of an individual.
The general anatomy closely resembles that of _Cephalodiscus_.[49] There
are five body-cavities and a notochord. Collar-pores exist, but
proboscis-pores and gill-slits have not been described. The dorsal region
of the collar bears only a single pair of arms.
ORDER III. PHORONIDEA.
The structure and development of _Phoronis_ (Fig. 13), have already been
described in Vol. II.[50] of this series; and Masterman's investigations,
then published in a preliminary form only, are there alluded to. Since then
this author has published fuller accounts[51] of his results, which, if
substantiated, {28}would indicate a near relationship between
_Cephalodiscus_ and _Phoronis_.
_Phoronis_ is a small tubicolous animal, of gregarious habits, which has
usually been regarded as related to the Gephyrea. Its body ends in a plume
of ciliated tentacles, which can be protruded from its tube, and the anus
is on the dorsal side, not far from the mouth. In both these respects it
agrees with _Cephalodiscus_, but a more striking similarity is asserted by
Masterman to exist between the latter and _Actinotrocha_, the larval stage
of _Phoronis_. The prae-oral ciliated hood (Fig. 14) of Actinotrocha is
regarded as the proboscis, and it contains a median cavity, traversed, like
that of Balanoglossus, by muscular fibres. The collar is the region between
the constricted neck and an oblique line, parallel to and immediately
behind the series of tentacles, which thus belong to the collar. This
division has a collar-cavity which is said to be distinct from the
prae-oral cavity, and is separated by a septum from the posterior
body-cavity. Its dorsal epidermis contains the central nervous system
(_n.s_), which is connected with a system of nerves resembling those of
Balanoglossus. A median diverticulum of the alimentary canal of this part
may be compared with the notochord of that animal, but there are no
gill-slits.
[Illustration: FIG. 13.—_Phoronis buskii_, M‘Intosh, Philippine Islands, x
about 2. (After M‘Intosh, from Shipley.)]
The remainder of the body of Actinotrocha corresponds with the trunk of
Balanoglossus. Its body-cavity is distinct from that of the collar, and is
divided by a ventral mesentery, though not by a dorsal mesentery. A
noteworthy fact is that both Actinotrocha and Tornaria swim by means of a
ring of strong cilia or membranellae[52] which surrounds the anus.
{29}[Illustration: FIG. 14.—_Actinotrocha_-larva of _Phoronis_. _a_, Anus;
_b.c^1_, _b.c^2_, _b.c^3_, first, second and third body-cavities; _c_,
circular nerve, running in the posterior boundary of the collar,
immediately behind the ring of tentacles; _c.r_, ciliated ring; _d_,
diverticulum (paired) of alimentary canal; _m_, mouth; _n.s_, central
nervous system; _p_, nerve running round the ventral border of the
proboscis; _s_, sense-organ; _s.s_, subneural sinus, a vascular space whose
hind wall is constituted by the front boundary of _b.c^2_, its front wall
being formed by the hind wall of _b.c^1_; in this region is seen a median
outgrowth of the alimentary canal, which may be compared with the notochord
of _Cephalodiscus_, or of the young Tornaria (cf. Morgan, _J. Morphol_. v.
1891, Plate xxvi. Fig. 40.) (After Masterman.)]
Important memoirs on the structure of Actinotrocha have recently been
published by Ikeda,[53] de Selys Longchamps,[54] Goodrich,[55] and
Schultz,[56] who criticise many of Masterman's statements. While it is
admitted on all sides that an oblique septum following the line of the
bases of the tentacles completely subdivides the body-cavity, Masterman's
account of the anterior cavities is not confirmed, the spaces indicated by
_b.c^1_ and _b.c^2_ in Fig. 14 being stated to be really continuous with
one another, while the "subneural sinus" (_s.s_) is regarded as a part of
this space. It appears, however, from the account given by Ikeda, and
followed by Goodrich, that the old Actinotrocha has two distinct spaces in
front of the septum. The first of these corresponds with _b.c^1_ + most of
_b.c^2_ in Fig. 14, and is continuous with the cavities of the larval
tentacles. Into it project the blind ends of the larval excretory organs,
which, according to Goodrich, bear numerous "solenocytes" similar to those
described by the same author in Amphioxus and in Polychaet worms (Fig. 79,
p. 127). The second cavity is a relatively small crescent (not shown in
Fig. 14), lying on the anterior face of the septum, {30}the tips of the
crescent nearly meeting dorsally, so as to constitute an almost complete
ring following the bases of the tentacles, into each of which it gives off
a blind outgrowth. At the metamorphosis, the crescentic space becomes the
prae-septal body-cavity and the cavities of the tentacles of the adult, the
circular blood-vessel of which is formed from the remains of the large
prae-septal space of the larva. Schultz, in calling attention to the fact
that both _Phoronis_ and its larva have a striking power of regenerating
lost parts, confirms the conclusion that this animal belongs to the
Hemichordata. He gives reasons, however, for believing that it is in the
adult _Phoronis_ rather than in the larval Actinotrocha that it is possible
to discover the most satisfactory evidence of this affinity.
The metamorphosis[57] of Actinotrocha is very remarkable, and is
accompanied by the eversion of a ventral ingrowth of the body-wall. A loop
of the alimentary canal passes into this eversion, which becomes the main
part of the body of the adult; and the anus is thereby brought relatively
nearer the mouth than in the larva. The occurrence of this process may help
to explain the position of the anus in the Pterobranchia.
AFFINITIES OF THE HEMICHORDATA.—There can be no doubt that some of the
resemblances, in structure and in development, between Balanoglossus and
certain Vertebrates are extremely striking. The view that Balanoglossus is
related to the ancestors of Vertebrates[58] appears to exclude other
views[59] which have been suggested with regard to the same question. The
Balanoglossus-theory does not explain the similarity between the
segmentation and the excretory systems of Vertebrates and Chaetopods; but,
on the contrary, there are important characters which Vertebrates share
with Balanoglossus but with no other "Invertebrates." Of these the most
important appear to be the resemblances between the gill-slits and
gill-bars of Balanoglossus and Amphioxus; the position, structure and mode
of development of the central nervous system; and the presence of a
structure in the Hemichordata, which may be regarded as the notochord.
{31}There are other points in which Balanoglossus specially resembles
Amphioxus, such as the early development, the mode of formation of the
body-cavities,[60] and the presence of numerous generative organs.
All these, taken together, make it necessary to consider carefully the
claims of Balanoglossus to relationship with the ancestors of Vertebrates
in making any speculations on this interesting problem.
However improbable it may appear at first sight, it is possible to hold the
view that Balanoglossus is related at the same time to Vertebrates and to
Starfishes and other Echinoderms. The similarity between a young Tornaria
and a young Bipinnaria-larva of a Starfish is so great as to have misled
even Johannes Müller. The more obvious resemblances are the almost
identical course of the longitudinal ciliated band in the young stages, and
the presence of a dorsal pore. The Echinoderm-larva is not, however,
provided with eye-spots, nor has it the posterior, or transverse, ciliated
band of Tornaria.
Recent studies on the development of Echinoderms[61] have made it probable
that the five body-cavities of Balanoglossus are represented in the larvae
of those animals; and this materially strengthens the probability of the
view that the respective adults are also allied.[62] It may be added that
the relationship which appears to be indicated is between Balanoglossus and
the bilateral ancestors from which the radially-symmetrical Echinoderms are
probably descended.
In comparing the Enteropneusta with the Pterobranchia, the disproportionate
size of the trunk of Balanoglossus may perhaps be explained by assuming
that the region of the third body-cavities has been enlarged since
Balanoglossus branched off from the ancestral stock.[63] The approximation
of the anus to the mouth in Pterobranchia is perhaps the result of their
tubicolous habits.[64] In the position of the central nervous system in the
skin of the collar, _Cephalodiscus_ appears to be more primitive {32}than
Balanoglossus, as has been pointed out by Morgan.[65] It is not impossible
that the presence of one pair of gill-slits in _Cephalodiscus_ indicates
that this animal diverged from the ancestors of Balanoglossus before the
gill-slits were metamerically repeated.
ASCIDIANS AND AMPHIOXUS
BY
W. A. HERDMAN, D.SC. (EDINB.), F.R.S.
Professor of Natural History in the University of Liverpool
{35}CHAPTER II
TUNICATA (ASCIDIANS AND THEIR ALLIES)
INTRODUCTION—OUTLINE OF HISTORY—STRUCTURE OF A TYPICAL ASCIDIAN—EMBRYOLOGY
AND LIFE-HISTORY
The TUNICATA are marine animals found in practically all parts of the sea,
and at all depths. They extend from the Arctic and Antarctic regions to the
tropical waters, and from the littoral zone down to the abyssal depths of
over three miles. They are abundant in British seas. They vary greatly in
shape and colour, and range in size from an almost invisible hundredth of
an inch to large masses a foot or more in diameter. And yet most Tunicata
have a characteristic appearance by which they can be readily distinguished
from other animals. They form a well-defined group, with definite
anatomical characters, and there are no known forms intermediate between
them and other groups. The Tunicata were formerly regarded as constituting,
along with the Polyzoa and the Brachiopoda, the Invertebrate Class
"MOLLUSCOIDEA." They are now known to be a degenerate branch of the lower
CHORDATA, and to be more nearly related to the Vertebrata than to any group
of Invertebrates.
Tunicata occur either fixed or free, solitary, aggregated or in colonies
(see Fig. 27, p. 64). The fixed forms, found on the sea-bottom, are usually
termed "Ascidians," those that are solitary or merely aggregated being
"Simple Ascidians" or Monascidiae, and those that are organically united
into a colony being "Compound Ascidians" or Synascidiae. The colonies have
been produced by budding, a process which is very general in the group, and
the members of the colony are conveniently known as "Ascidiozooids." Some
exhibit {36}alternation of generations, and all pass through remarkable
changes in their life-history, nearly all of them undergoing a
retrogressive metamorphosis.
OUTLINE OF HISTORY.
More than two thousand years ago Aristotle gave a short account of a Simple
Ascidian under the name of _Tethyum_. He described the appearance and some
of the more important points in the anatomy of the animal. From that time
onwards comparatively little advance was made until Schlosser and Ellis, in
a paper on _Botryllus_, published in the _Philosophical Transactions_ of
the Royal Society for 1756, first brought the Compound Ascidians into
notice. It was not, however, until the commencement of the nineteenth
century, as a result of the careful anatomical investigations of Cuvier[66]
upon the Simple Ascidians, and of Savigny[67] upon the Compound Ascidians,
that the relationship between these two groups of Tunicata was conclusively
demonstrated. Up to 1816, the date of publication of Savigny's great work,
the few Compound Ascidians previously known had been generally regarded as
Alcyonaria or as Sponges; and although many new Simple Ascidians had been
described by O. F. Müller[68] and others, their internal structure had not
been investigated. Lamarck[69] in 1816, chiefly as the result of the
anatomical discoveries of Savigny and Cuvier, instituted the class
TUNICATA, which he placed between the Radiata and the Vermes in his system
of classification. The Tunicata included at that time, besides the Simple
and the Compound Ascidians, the pelagic forms _Pyrosoma_, which had been
first made known by Péron in 1804, and _Salpa_ described by Forskål in
1775.
Chamisso, in 1819, made the important discovery that _Salpa_ in its
life-history passes through the series of changes which were afterwards
more fully described by Steenstrup in 1842 as "alternation of generations";
and a few years later Kuhl and Van Hasselt's investigations upon the same
animal resulted in the discovery of the alternation in the directions in
which the wave of contraction passes along the heart, and in which the
{37}blood circulates through the body. It has since been found that this
observation holds good for all groups of the Tunicata. In 1826, H.
Milne-Edwards[70] and Audouin made a series of observations on living
Compound Ascidians, and amongst other discoveries they found the
free-swimming tailed larva and traced its development into the young
Ascidian.
In 1845, Carl Schmidt[71] first announced the presence in the test of some
Ascidians of "tunicine," a substance very similar to cellulose; and in the
following year Löwig and Kölliker[72] confirmed the discovery, and made
some additional observations upon this substance and upon the structure of
the test in general. Huxley,[73] in an important series of papers published
in the _Transactions_ of the Royal and Linnean Societies of London from
1851 onwards, discussed the structure, embryology, and affinities of the
pelagic Tunicates, _Pyrosoma_, _Salpa_, _Doliolum_ and _Appendicularia_.
These important forms were also investigated about the same time by
Gegenbaur, Vogt, H. Müller, Krohn, and Leuckart.
The most important epoch in the history of the Tunicata is the date of the
publication of Kowalevsky's celebrated memoir[74] upon the development of a
Simple Ascidian. The tailed larva had been previously discovered and
investigated by several naturalists, notably by H. Milne-Edwards,[75] P. J.
van Beneden, and Krohn; but its minute structure had not been sufficiently
examined, and the meaning of what was known of it had not been understood.
It was reserved for Kowalevsky in 1866 to demonstrate the striking
similarity in structure and in development between the larval Ascidian and
the Vertebrate embryo. He showed that the relations between the nervous
system, the notochord, and the alimentary canal are practically the same in
the two forms, and have been brought about by a very similar course of
embryonic development. This discovery clearly indicated that the Tunicata
are closely allied to Amphioxus and the Vertebrata, and that the tailed
larva represents the primitive or ancestral form from which the adult
Ascidian has been evolved by degeneration. This led naturally to the view
usually accepted at the present day, that the group is a degenerate
{38}side-branch from the lower end of the phylum CHORDATA, which includes
the Tunicata (or Urochordata), Balanoglossus and its allies (Hemichordata),
Amphioxus (Cephalochordata), and the Vertebrata (or Craniata). Kowalevsky's
great discovery has since been confirmed and extended to all other groups
of the Tunicata by Kupffer,[76] Giard, and others.
In 1872 Fol[77] added largely to the knowledge of the Appendiculariidae,
and Giard[78] to that of the Compound Ascidians. The latter author
described a number of new forms and remodelled the classification of the
group. The most important additions which have been made to the Compound
Ascidians since Giard's work have been the species described by von
Drasche,[79] from the Adriatic, and those discovered by the "Challenger"
expedition.[80] The structure and the systematic arrangement of the Simple
Ascidians have been discussed of recent years mainly by Alder[81] and
Hancock, Heller,[82] Lacaze-Duthiers,[83] Traustedt,[84] Roule, Hartmeyer,
Sluiter[85] and Herdman.[86] In 1874 Ussoff investigated the minute
structure of the nervous system and of the underlying gland, which was
first discovered by Hancock, and showed that the gland has a duct which
communicates with the front of the branchial sac or pharynx by an aperture
in the dorsal (or "olfactory") tubercle. In an important paper published in
1880, Julin[87] drew attention to the similarity in structure and relations
between this gland and the "hypophysis cerebri" of the Vertebrate brain,
and insisted upon their homology. Metcalf has recently added further to our
knowledge on this and related matters.
The Thaliacea or pelagic Tunicata have of late years been the subject of
several very important memoirs. The researches {39}of Todaro, Brooks,[88]
Salensky,[89] Seeliger,[90] Korotneff,[91] and others have elucidated the
embryology, the gemmation and the life-history of the Salpidae; and
Grobben, Barrois,[92] and more especially Uljanin,[93] have elaborately
worked out the structure and the details of the complicated life-history of
the Doliolidae. Finally we owe to the labours of Metschnikoff, Kowalevsky,
Giard, Hjort, Seeliger, Ritter, Van Beneden and Julin, much detailed
information as to development and life-history, the process of gemmation
and the formation of colonies, which has added greatly to our knowledge of
the position and affinities of the Tunicata and of their natural
classification.
STRUCTURE OF A TYPICAL ASCIDIAN.
If a typical "Simple Ascidian," such as the common British _Ascidia
mentula_ (Fig. 15), or _Ascidia virginea_, be examined alive and expanded
in sea-water it will be seen to bear on the upper surface two short
projections, each terminated by a wide tubular opening, through which the
animal, when touched, can emit jets of water with considerable force—thus
accounting for the popular name "sea-squirts." The rest of the body is
covered by the dull grey tough cuticular outer "test" or "tunic" (hence
TUNICATA) by means of which the animal is attached to a rock or other
foreign body. One of the tubular openings, the mouth or "branchial
aperture," is terminal, and indicates the morphological anterior end; it is
surrounded by eight lobes. The other opening, the cloaca or "atrial
aperture," is on the dorsal edge, from one-third to one-half way down the
body, and is bounded by six lobes only; consequently the two apertures, and
so the ends of the body, can be distinguished externally by the number of
lobes—an important matter. The area of attachment is usually the posterior
part of the left side; in Fig. 15 the animal is seen from the right hand
side.
If a little carmine-powder, or some other insoluble particles be scattered
in the water in which the Ascidian is living, the {40}particles will be
seen to converge to the branchial aperture and be sucked in by the inhalent
current entering the body. After a short interval a certain proportion of
the particles will be shot out from the atrial aperture with the exhalent
current.
These particles have passed through the pharyngeal portion of the
alimentary canal and the cloacal passages, with the water used in
respiration, but a considerable amount of such particles taken in with the
water do not reappear, as they are retained by the nutritive organs and
pass along the remainder of the alimentary canal with the food. The current
of water passing in at the branchial and out at the atrial aperture is of
primary importance in the life of the Ascidian. Besides serving for
respiratory purposes it conveys all the food into the body and removes
waste matters both intestinal and renal, and also expels the reproductive
products from the body.
[Illustration: FIG. 15.—_Ascidia mentula_ Linn. from the right side
(natural size), Loch Fyne, N.B.; _Br_, Branchial aperture; _At_, atrial
aperture. Arrows show the direction of the water currents.]
THE TEST.—The test is notable amongst animal structures for containing
"tunicine," a substance which appears to be identical in composition, and
in behaviour under treatment with various reagents, with cellulose. It is
cartilaginous in appearance and consistency, and to some extent in
structure, as it consists of a clear (or in some cases fibrillated) matrix
in which are embedded many corpuscles or cells. It is the matrix that
contains the cellulose, which may form over sixty per cent by weight of the
entire test. As the test is morphologically a cuticle, being a secretion on
the outer surface of the ectoderm (Fig. 16, _ec_), the cells it contains
have immigrated to it from the body, and it has recently been shown that
many of these are mesodermal cells (leucocytes or connective tissue
wandering cells, amoebocytes, and in some cases embryonic "kalymmocytes,"
or {41}egg-follicle cells, see below, p. 56), which have passed through the
ectoderm. This process commences in the larval state with the migration of
mesenchyme cells from the blastocoele through the epiblast. Ectoderm cells,
and possibly also some primitive endoderm cells, also take part in forming
the test. Many of these cells in the test remain small and simple, as the
fusiform and stellate test-cells; some become pigment-cells, while others
enlarge and become vacuolated to form the large (up to 0.15 mm. in
diameter) vesicular or "bladder" cells—this is especially the case in the
outer layer of the test in _Ascidia mentula_ (see Fig. 17, _bl_) where
there are innumerable clear vesicles, each surrounded by a thin film of
protoplasm and having the nucleus still visible at one point of the
surface. In some of the Tunicata the test-cells produce calcareous spicules
of various shapes (see below, p. p. 86).
[Illustration: FIG. 16.—Diagrammatic section through test and mantle of
_Ascidia_ to show the relations of ectoderm to body-wall and cuticle.
_bl.c_, Bladder-cells; _bl.s_, blood-sinus; _c.t.c_, connective tissue
cells; _ec_, ectoderm; _mes.c_, wandering mesoblast cells; _m.f_, muscle
fibres; _t.c_, test-cells; _t.v_, "vessel" of the test."]
The test also becomes organised by the growth into it of the so-called
"vessels." These are outgrowths of the body-wall covered by ectoderm and
containing prolongations of blood-channels from the connective tissue of
the "mantle" (body-wall). Fig. 16, _t.v_ shows such an outgrowth, and
exhibits the general relations of test (cuticle), ectoderm, and mesoderm.
It also explains how it is that the blood-channel being pushed out as a
{42}loop gives rise to the double or paired "vessels" seen branching
through the test (see Fig. 17, _v_). The two vessels of a pair are one
blood-channel imperfectly divided by a connective-tissue septum. The blood
courses out along one side, round the communication in a "terminal knob" at
the end, and back down the other side. The "terminal knobs" are very
numerous, and form a marked feature in the outer layer of the test (Fig.
17, _t.k_); in some cases (_Culeolus murrayi_), they probably form an
accessory organ of respiration, while in others (Botryllidae), they pulsate
and aid in keeping up the circulation.
The ectoderm is a simple epithelial layer (Fig. 16, _ec_). It is turned in
for a short distance at the branchial aperture (mouth), and atrial aperture
(cloaca), as a short stomodaeum and proctodaeum respectively, lined in each
case by a delicate prolongation of the test.
[Illustration: FIG. 17.—Section through the surface layer of test of
_Ascidia mentula_, × 50. _bl_, Bladder cells; _t.c_, test cell; _t.k_,
terminal knobs of vessels; _v_, vessels of test.]
Fig. 24, A, p. 52, shows the relations of ectoderm, mesoderm, and endoderm
in a section through the antero-dorsal part of the body. The cavity marked
_p.br_ is a portion of the atrial cavity lined by ectoderm, and must not be
confounded with a coelom. The absence of a true coelom in the mesoderm will
be noticed in this and other figures, and yet the Tunicata are Coelomata,
although it is very doubtful whether the enterocoel which has been
described in the development of some is ever found. The coelom is in any
case largely suppressed later, and is only represented in the adult by the
pericardium and by small cavities in the renal and reproductive organs and
ducts.
BODY-WALL AND CAVITIES OF THE BODY.—The name "mantle" is given to the
ectoderm with the parietal mesoderm which form the body-wall inside the
test. It is largely formed of connective {43}tissues—both homogeneous and
fibrous—with cells, blood-sinuses, and many muscle-bundles large and small
running circularly, longitudinally, and obliquely, and interlacing in all
directions (Fig. 18, _m_). The muscles are all formed of very long fusiform
non-striped fibres. The mantle in some Ascidians is often brilliantly
pigmented—red, yellow and opaque white, the coloured cells being exactly
like those found in the blood.
[Illustration: FIG. 18.—Dissection of _Ascidia_, from right side, to show
anatomy. _a_, Anus; _At_, atrial aperture; _Br_, branchial aperture;
_br.s_, _br.s′_, branchial sac; _end_, endostyle; _g.d_, genital ducts;
_gon_, ovary; _hyp_, neural gland; _hyp.d_, the duct leading to dorsal
tubercle; _m_, mantle; _n.g_, ganglion; _oes_, oesophagus; _p.br.c_,
peribranchial cavity; _ren_, renal vesicles; _st_, stomach; _t_, test;
_tn_, tentacles; _ty_, typhlosole.]
The mantle forms two well-marked siphons or short wide tubes, which lead in
from the branchial and atrial apertures. These are surrounded by strong
sphincter muscles,[94] and are lined by the invaginated ectoderm and test.
The one leads into the branchial sac or modified pharynx, and the other
into the atrial or peribranchial cavity (see Fig. 18, and Fig. 19, _p.br_).
Figs. 18 and 19 show the relations of the branchial and peribranchial
cavities to one another. The peribranchial cavity {44}opens to the exterior
dorsally by the atrial aperture, forms the cloaca along the dorsal edge of
the body, and has extensions laterally on each side of the branchial sac,
with the interior of which it is placed in communication by the secondary
gill-slits or "stigmata" (Fig 19, _sg_). Along the ventral edge the mantle
is united to the wall of the branchial sac, and it is only this union (Fig.
19, _end_) that prevents the peribranchial cavity from completely
surrounding the branchial sac.
The following list of the cavities present in the body of the adult
_Ascidia_ may be useful at this point:—
1. The alimentary canal, including the branchial sac. This is derived from
the archenteron of the embryo, is lined throughout by endoderm, and the
system of cavities of the intestinal gland is to be regarded merely as an
outgrowth from the alimentary canal.
2. The peribranchial (atrial) cavity, derived from two lateral ectodermal
invaginations which join dorsally to form the cloaca and open to the
exterior by the atrial aperture.
3. The original embryonic segmentation cavity (blastocoele) remains, where
not obliterated by the development of the mesodermal connective tissue, as
the irregular system of blood spaces, with its outgrowths in test and
branchial sac. The heart, which has differentiated muscular walls, becomes
secondarily connected at its ends with these blood spaces.
4. The pericardium and epicardium (see p. 83) originate as outgrowths from
the archenteron. They may therefore be regarded as enterocoelic spaces. The
pericardium becomes completely closed off and separated from the alimentary
canal. The epicardium may form paired tubes of great length, and may
remain permanently connected with the branchial sac.
5. The cavities of the renal vesicles and of the gonads and ducts are
spaces formed in the mesoblast. They have been variously interpreted:—
(_a_) As of the same nature as the blood spaces (blastocoelic), or
(_b_) As formed by a splitting of the mesoblast (coelomic).
6. The cavity of the neural gland and of its duct opening at the dorsal
tubercle is derived from the primitive dorsal neural tube of the embryo,
and so may be regarded as a part of the lumen of the cerebro-spinal nervous
system.
TENTACLES, ETC.—The branchial aperture leads through the {45}branchial
siphon into the branchial sac. At the base of the siphon, just about the
line of junction of the ectoderm of the stomodaeum with the endoderm of the
mesenteron, is placed a circle of simple hair-like tentacles (Fig. 18,
_tn_) which stand out at right angles to the wall, and more or less
completely meet in the centre to form a delicate, sensory grid or sieve
through which all the water entering the body has to pass. These tentacles
not only act mechanically, but are also sensitive although only scattered
sensory cells, and no specially differentiated sense-organs are found upon
them. Behind the tentacles lies the plain, or papillated, prebranchial zone
(Fig. 21, _p.br.z_), bounded behind by a pair of parallel and closely
placed ciliated ridges with a groove between—the peripharyngeal bands—which
encircle the anterior end of the branchial sac.
[Illustration: FIG. 19.—Semi-diagrammatic transverse section of _Ascidia_,
passing through the atrial aperture, seen from anterior surface, left side
uppermost. _At_, Atrial aperture; _at.l_, atrial lobe; _Br.s_, branchial
sac; _cl_, cloaca; _con_, connective; _d.bl.s_, dorsal blood-sinus; _d.l_,
dorsal lamina; _end_, endostyle; _g.d_, genital ducts; _i, i′_, intestine;
_l.v_, interstigmatic vessel; _m_, mantle; _m.b_, muscle-bundles; _ov_,
ovary; _p.br_, peribranchial cavity; _r_, rectum; _ren_, renal vesicles;
_sg_, stigmata; _sph_, atrial sphincter; _t_, test; _tr_, transverse
vessel; _ty_, typhlosole; _v.bl.s_, ventral blood-sinus.]
The branchial sac is very large—much the largest organ of the body—and
extends almost to the posterior end of the body, while the rest of the
alimentary canal lies upon its left side. The food particles, consisting of
microscopic plants and animals, are carried in through the branchial
aperture by the current of water, but most of them do not pass out through
the gill-slits to the atrium, being entangled in the viscid mucus which
passes by ciliary action along the groove between the peripharyngeal bands.
{46}ENDOSTYLE.—The mucus just referred to is produced in the long
canal-shaped gland called the endostyle or hypobranchial groove, which runs
along the entire ventral edge of the branchial sac (Fig. 18, _end_). The
sides, and especially the floor of the endostyle, are richly ciliated,
while there are four (or six) strongly-marked, peculiarly-shaped glandular
tracts, two (or three) on each side (Fig. 20, _gl_) running along its
length, and separated by areas of closely-packed fusiform cells with short
cilia, amongst which are found some bipolar sensory cells.
[Illustration: FIG. 20.—Transverse section of the endostyle of _Ascidia
mentula_, × 350. _bl.s_, Blood-sinus; _end.l_, lips of the endostyle; _gl_,
glandular tracts; _i.l_, internal longitudinal bar; _l.v_, interstigmatic
vessels; _m_, mantle; _p.br_, peribranchial cavity; _sg_, stigmata;
_v.bl.s_, ventral blood-sinus.]
This organ corresponds to the hypopharyngeal groove of Amphioxus and the
median part of the thyroid gland of Vertebrata. It is interesting to notice
that the (at least) four longitudinal tracts of gland-cells are of
remarkable constancy, being found not only in all groups of Tunicata,
including even the pelagic, tailed Appendicularians, but also in Amphioxus
and in the young thyroid gland of the Ammocoete. When, in Ascidians, a
third marginal glandular tract is added it has a different appearance from
the two characteristic tracts. The mucus is carried forward by the action
of the large floor-cilia of the endostyle (Fig. 20) to the groove between
the peripharyngeal bands, and after encircling the anterior end of the
branchial sac and collecting the food particles, it passes backwards along
the dorsal edge {47}of the branchial sac to the oesophagus, guided by a
membranous fold, the dorsal lamina (Fig. 21, _d.l_), which is more or less
ridged or corrugated, and may be armed with marginal tags or even replaced
by larger processes (the "languets") in some species of Ascidians. In the
living animal the lamina has its free edge curved to the right hand side in
such a manner as to constitute a fairly perfect tube along which the train
of food passes.
[Illustration: FIG. 21.—Antero-dorsal part of pharynx in _Ascidia mentula_,
× 15. _br.s_, Part of branchial sac; _d.l_, dorsal lamina; _d.t_, dorsal
tubercle; _p.br.z_, prebranchial zone; _p.p_, peripharyngeal bands; _sph_,
sphincter of branchial aperture; _tn_, tentacle.]
BRANCHIAL SAC.—Thus we have the dorsal lamina (or the languets) along the
dorsal edge, the endostyle along the ventral edge, and the peripharyngeal
bands around the anterior end. The wall of the branchial sac itself is
penetrated by a large number of channels through which blood flows. Some of
these run in one direction and some in another, so as to form complicated
networks, which differ greatly in their arrangement in different Ascidians.
Between these blood-channels there are clefts ("stigmata"), the secondary
or subdivided gill-slits, by means of which the current of water passes
from the branchial sac to the large external peribranchial or atrial
cavity. All the stigmata (of which there may be several hundred thousand)
in the wall of the branchial sac are bounded by cubical or columnar
epithelial cells, which are ciliated. These cilia, so long as the animal is
alive, are in constant motion, so as to drive the water onwards, and it is
this constant ciliary action in the walls of the branchial sac that gives
rise to the all-important current of water streaming through the body. In
addition to the stigmata there are generally one or two much larger
elongated slits (Garstang's {48}pharyngo-cloacal slits) placed close to the
dorsal lamina and leading direct to the cloaca.
[Illustration: FIG. 22.—A mesh of the branchial sac of _Ascidia_, seen A,
from inside; B, in horizontal section. _c.d_, Connecting duct; _h.m_,
horizontal membrane; _i.l_, internal longitudinal bars; _l.v_,
interstigmatic vessels; _p_, _p′_, papillae; _sg_, stigmata; _tr_,
transverse vessels.]
Fig. 22 shows a small part of the wall of the branchial sac, in which it
may be seen that the bars containing the blood-channels are arranged in
three regular series:—(1) The "transverse vessels" which run horizontally
round the wall and open at their dorsal and ventral ends into large median
longitudinally running tubes, the dorsal blood-sinus (or "dorsal aorta")
behind the dorsal lamina, and the ventral blood-sinus (or "branchial
aorta") beneath the endostyle; (2) the fine longitudinal or "interstigmatic
vessels" which run vertically between adjacent transverse vessels and open
into them, and which therefore bound the stigmata; and (3) the "internal
longitudinal bars" which run vertically, in a plane internal to that of the
transverse and fine longitudinal vessels. These bars (Fig. 22, _i.l_)
communicate with the transverse vessels by short side branches where they
cross, and at these points are prolonged into the cavity of the sac in the
form of hollow papillae. In some Ascidians (e.g. _Corella_ and most of the
Molgulidae) the interstigmatic vessels are curved so that the stigmata form
more or less complete spirals (see Figs. 35 and 41). In some species of
_Ascidia_, and other Ascidians, the interstigmatic vessels are inserted
into the transverse vessel in an undulating course in place of the straight
line seen in Fig. 22, B, _l.v_, the result being that the stigmatic part of
the wall of the branchial sac seems to be folded or thrown into microscopic
crests and troughs. This is known as "minute plication." In some cases,
again (Cynthiidae), the whole wall of the sac is pushed inwards at
intervals to form large folds visible to the eye (see Fig. 36, A and B).
The intersections of the internal longitudinal bars with the transverse
vessels divide up the inner surface of the branchial {49}sac wall into
rectangular areas called "meshes." One such mesh, containing eight stigmata
in a row, is seen in Fig. 22, A. The internal longitudinal bars bear
papillae at the angles of the meshes, and occasionally in intermediate
positions. There are frequently horizontal membranes (Fig. 22, B, _h.m_)
attached to the transverse vessels between the papillae. There are many
"connectives" running from the outer wall of the branchial sac to the
mantle outside, and allowing the blood in the transverse vessels to
communicate with that in the sinuses of the mantle (see Fig. 19, _con_).
HEART AND CIRCULATION.—It is one of the notable features of the Tunicata
that the circulation is not constant in direction, but is periodically
reversed.
The blood of Ascidians is in the main transparent, but usually contains
certain pigmented corpuscles in addition to many ordinary leucocytes or
colourless amoeboid cells. The pigment in the coloured cells may be red,
yellow, brown, or in some cases blue or opaque white. The blood may reach
the branchial sac either from the dorsal or from the ventral median sinus
according to the direction in which the heart is beating at the moment (see
below); and it is a most interesting and beautiful sight to see the
circulation of the variously coloured corpuscles through the transparent
vessels, and the lashing of the cilia along the edges of the neighbouring
stigmata in a small Ascidian under the microscope.
In _Ascidia_ (Fig. 23) the heart is an elongated fusiform tube placed on
the ventral and posterior edge of the stomach, projecting into a space (the
pericardium) which is a part of the original coelom, the remainder of which
is represented in the adult by the reproductive and renal cavities. The
wall of the heart is continuous along one edge with that of the
pericardium, and the heart is to be regarded as a tubular invagination of
the pericardial wall, shutting in a portion of the surrounding space (the
blastocoel of the embryo), and having open ends which communicate with the
large blood sinuses leading to the branchial sac, to the viscera, and to
the body-wall and test. The cavity of the heart is not divided and there
are no valves. Its wall is formed of a single layer of epithelio-muscular
cells, the inner, muscular, ends of which are cross-striated fibres running
round the heart—the only striated muscular tissue found in the body. Waves
of {50}contraction pass along the heart from end to end, first for a
certain number of beats in one direction, and then, after an interval, in
the other. If a small or young _Ascidia_ be placed alive, left side
uppermost, in a watch-glass or small trough of sea-water, and examined with
a low power of the microscope, the heart will be readily seen near the
posterior end of the transparent body. It will be noticed that the
"beating" looks like successive waves of blood pressed through the tubular
heart from one end to the other by its contractions. After watching the
waves passing, let us say, from the right hand end of the heart to the left
for about a minute and a half (perhaps 60 or 80 to 100 beats), it will be
seen that they gradually become slower and then stop altogether. But after
seven or eight seconds a faint wave of contraction will start from the
_left_ end of the heart and pass over it to the right; and this will be
followed by larger ones for a minute and a half, and then again a pause
will occur and the direction change. It has been suggested that the cause
of this remarkable reversal may possibly be that the heart being on the
ventral vessel, which is wider than the corresponding dorsal trunk, pumps
the blood into either the lacunae of the branchial sac or those of the
viscera in greater volume than can possibly get out through the smaller
branchio-visceral vessel in the same time, the result being that the
lacunae in question soon become engorged, and by back pressure cause the
stoppage, and then reversal of the beat. The absence of any valves in the
heart to regulate the direction of flow obviously facilitates this
alternation of the current.
The larger channels through which the blood flows may be lined with a
delicate endothelium, but the smaller passages are merely spaces in the
connective tissue. The heart, although anatomically a "ventral vessel,"
runs in the main dorso-ventrally. The blood-channel leaving the ventral end
of the heart is the "branchio-cardiac vessel" (Fig. 23, _b.c_). This gives
off a branch which, along with a corresponding branch from the
"cardio-visceral" vessel (_c.v_) at the other end of the heart, goes to the
test, and then runs along the ventral edge of the branchial sac as the
branchial aorta (_b.a_), external to the endostyle, communicating laterally
with the ventral ends of all the transverse vessels of the branchial sac.
The cardio-visceral vessel (Fig. 23, _c.v_) after giving off its branch to
the test breaks up into a number of sinuses which {51}ramify over the
alimentary canal and the other viscera. These visceral lacunae finally
communicate with a third great sinus, the "branchio-visceral" vessel
(_b.v_) which runs forward along the dorsal edge of the branchial sac as
the dorsal aorta (_d.a_), externally to the dorsal lamina, and joins the
dorsal ends of all the transverse vessels of the branchial sac. Besides
these three chief systems—the branchio-cardiac, the cardio-visceral, and
the branchio-visceral—(see Fig. 23), there are numerous lacunae in all
parts of the body by means of which anastomoses are established between the
different currents of blood.
[Illustration: FIG. 23.—Diagrammatic dissection of _Ascidia_, from left
side, to show course of circulation. Front part of branchial sac opened,
back part covered by viscera. _b.a_, Branchial (ventral) aorta; _b.c_,
branchio-cardiac vessel; _b.v_, branchio-visceral vessel; _c.v_,
cardio-visceral vessel; _d.a_, dorsal aorta; _ht_, heart. _A_, anterior;
_P_, posterior; _D_, dorsal; _V_, ventral.]
When the heart contracts ventro-dorsally the course of the circulation is
as follows:—the blood which is flowing through the vessels of the branchial
sac is collected in an oxygenated condition in the branchio-cardiac vessel,
and after receiving a stream of blood from the test enters the ventral end
of the heart. It is then propelled from the dorsal end into the
cardio-visceral vessels, and so reaches the test and the digestive and
other viscera; then, after circulating in the visceral lacunae it passes
into the branchio-visceral vessel in an impure condition, and is
distributed to the branchial vessels to be purified again. When the heart,
on the other hand, contracts dorso-ventrally, this course of the
circulation is reversed, the "veins" and "arteries" exchange functions, and
what a minute before was a "systemic," is now a "respiratory" heart. This
is a phenomenon without parallel in the animal kingdom.
{52}All the blood-spaces and lacunae are probably derived, like the cavity
of the heart, from the blastocoel of the embryo, and are not, like the
cavity of the pericardium, a part of the coelom (of endodermal origin).
NEURAL GLAND AND DORSAL TUBERCLE.—In the dorsal median line near the
anterior end of the body, and imbedded in the mantle on the ventral[95]
surface of the nerve-ganglion, there lies a small glandular mass—the neural
gland—which, as Julin first showed, there is some reason to regard as the
homologue of the hypophysis cerebri of the Vertebrate brain. Metcalf has
recently shown that the neural gland may be a double structure—partly
cerebral and partly stomodaeal—as in Vertebrates.
[Illustration: FIG. 24.—Antero-dorsal part of _Ascidia_ showing the
relations of the layers of the body, and of the nervous system. A, in
sagittal section; B, in transverse section. _d.bl.s_, Dorsal blood-sinus;
_d.l_, dorsal lamina; _d.n_, dorsal nerve; _d.t_, dorsal tubercle; _ect_,
ectoderm; _en_, endoderm; _e.p.br_, epithelium of peribranchial cavity;
_gl.d_, duct of subneural gland; _l.v_ points to the ciliated epithelium
covering a longitudinal vessel of branchial sac; _m_, mantle; _n_, nerve;
_n.g_, ganglion; _n.gl_, neural gland; _p.br_, peribranchial cavity;
_pp.b_, peripharyngeal bands; _sph_, branchial sphincter; _t_, _t′_, test;
_tn_, tentacle.]
The function of this gland is still somewhat mysterious. It may merely form
the viscid secretion which is carried along the peripharyngeal bands and
down the dorsal lamina. On the other hand, it has been suggested that the
function of the organ may possibly be renal, for the removal of nitrogenous
waste matters in the neighbourhood of the nervous system. Finally, it may
be a lymph gland.
{53}The neural gland, which was first noticed by Hancock, may be continued
backwards along with the dorsal nerve, and it communicates anteriorly by
means of a narrow duct with the front of the branchial sac (pharynx). The
opening of the duct is enlarged to form a funnel-shaped cavity (Fig. 24,
A), which may be folded upon itself, convoluted, or even broken up into a
number of smaller openings (see Fig. 43, p. 79), so as to form a
complicated projection called the dorsal tubercle, situated in the dorsal
part of the prebranchial zone. The dorsal tubercle in _Ascidia mentula_ is
somewhat horse-shoe shaped (Fig. 21, _d.t_); it varies in most Ascidians
(see Fig. 43) according to the genus and species, and in some cases in the
individual also. Sensory cells are found in the epithelium, and so it is
highly probable that besides being the opening of the duct from the neural
gland, this convoluted ciliated ridge may be a sense-organ for testing the
quality of the water entering the branchial sac.
NERVOUS SYSTEM AND SENSE-ORGANS.—The single elongated ganglion (Fig. 24,
_n.g_), in the median dorsal line of the mantle, between the branchial and
atrial siphons, is the only nerve-centre in _Ascidia_ and most other
Tunicata. It is the degenerate remains of the dorsal wall of the tubular
cerebro-spinal nervous system of the trunk-region of the tailed larval
Ascidian—the ventral wall opposite having given rise to the subneural
gland. The more posterior or spinal part of the larva has almost entirely
disappeared in most adult Tunicata. It persists, however, in the
Appendiculariidae, and traces of it have been found in the dorsal nerve
running backwards towards the oesophagus in some Ascidians (e.g.
_Clavelina_). It may be ganglionated in Molgulidae.
The ganglion has small rounded nerve-cells on its surface, and interlacing
nerve-fibres inside. It gives off distributory nerves at both ends (Fig.
24, A), which run through the mantle to the neighbourhood of the apertures,
where they divide up to supply the lobes and the sphincter muscles. The
only sense-organs are the pigment spots ("ocelli," formed of modified
ectoderm cells imbedded in red and yellow pigment), between the branchial
and atrial lobes, the tentacles at the base of the branchial siphon, and
probably the dorsal tubercle and the languets or dorsal lamina, in all of
which, as well as in the endostyle and peripharyngeal bands and in papillae
on the ectoderm and in the branchial sac, sensory cells have been found.
{54}These, considered as sense-organs, are all in a lowly-developed
condition. The larval Ascidians, on the other hand, have well-developed
intra-cerebral optic and otic sense-organs (see Fig. 26, p. 60), and in
some of the pelagic Tunicata, otocysts and pigment-spots are found in
connexion with the ganglion.
ALIMENTARY CANAL.—The mouth and pharynx (branchial sac) have already been
described. The remainder of the alimentary canal is a bent tube, which in
_A. mentula_ and most other Ascidians lies imbedded in the mantle on the
left side of the body, and projects into the peribranchial cavity (see
Figs. 18 and 19). The oesophagus leaves the branchial sac in the dorsal
middle line, near the posterior end of the dorsal lamina. It is a short
curved tube which leads ventrally to the large fusiform thick-walled
stomach, ridged internally. The intestine emerges from the ventral end of
the stomach and soon turns anteriorly, then dorsally, and then posteriorly,
so as to form a curve, the intestinal loop, in which the ovary lies, open
posteriorly. The intestine now curves anteriorly again, and from this point
runs nearly straight forward as the rectum, thus completing a second curve,
the rectal loop, in which the renal vesicles lie, open anteriorly. The wall
of the intestine is thickened internally to form the typhlosole (Fig. 18,
_ty_), a pad which runs along its entire length, so as to reduce the lumen
of the tube to a crescentic slit. The anus opens into the dorsal or cloacal
part of the peribranchial cavity near the atrial aperture. The walls of the
stomach are glandular, and most of the endoderm cells lining the tube are
ciliated. A system of delicate, microscopic, branched tubules with dilated
ends (the "refringent organ"), which ramifies over the outer wall of the
intestine, and communicates with the cavity of the stomach at the pyloric
end by means of a duct is probably a digestive gland. There is in _Ascidia_
no separate large gland to which the name "liver" can be applied, as in
some other Tunicata.
RENAL ORGAN.—A mass of large clear-walled vesicles which occupies the
rectal loop (Figs. 18 and 19, _ren_), and may extend over the adjacent
walls of the intestine, is a renal organ without a duct. Each vesicle is
the modified remains of a part of the primitive coelom or body-cavity, and
is formed of cells which eliminate nitrogenous waste matters from the blood
circulating in the neighbouring blood-lacunae, and deposit them in the
cavity of {55}the vesicle, where they form one or more concentrically
laminated concretions of a yellowish or brownish colour, sometimes coated
with a chalky deposit. These concretions contain uric acid, and in a large
Ascidian are very numerous. The nitrogenous waste products are thus
deposited and stored up in the renal vesicles in place of being excreted
from the body. In other Ascidians the renal organs may differ from the
above in position and structure; but in no case have they any excretory
duct, unless the neural gland is to be regarded as one of the renal
organs—which has not yet been proved.
REPRODUCTIVE ORGANS.—_Ascidia mentula_ is hermaphrodite, and the
reproductive organs lie with the alimentary canal, on the left side of the
body (Fig. 19, _ov_). The ovary is a ramified gland which occupies the
greater part of the intestinal loop. It contains a cavity which, along with
the cavities of the testis, is derived from an embryonic coelom; the ova
are formed from its walls, and fall when mature into the cavity. The
oviduct is continuous with the cavity of the ovary, and leads forward
alongside the rectum, finally opening near the anus into the peribranchial
cavity (Fig. 18, _g.d_). The testis is composed of a great number of
delicate, branched tubules, which ramify over the ovary and the adjacent
parts of the intestinal wall. These tubules terminate in ovate swellings.
Near the commencement of the rectum the larger tubules unite to form the
vas deferens, a tube of considerable size, which runs forward alongside the
rectum, and, like the oviduct, terminates by opening into the peribranchial
cavity close to the anus. The lumen of the tubules of the testis, like the
cavity of the ovary, is a part of the embryonic mesoblastic space, and the
spermatozoa are formed from the cells lining the wall. In some Ascidians
(certain Molgulidae and Cynthiidae), reproductive organs are present on
both sides of the body, and in others, as in _Polycarpa_, there are many
complete sets of both male and female systems attached to the inner surface
of the mantle on both sides of the body and projecting into the
peribranchial cavity.
EMBRYOLOGY AND LIFE-HISTORY OF A TYPICAL ASCIDIAN.
The eggs of Tunicata are for the most part of small size, nearly colourless
and transparent, and with little or no food-yolk. {56}In some, however
(such as some of the Cynthiidae, and some Compound Ascidians), the eggs are
larger, more opaque, and have a fair amount of food-yolk. Ova of this type
are not expelled from the body of the parent as ova, but are fertilised,
and remain in the atrial cavity or in a special diverticulum thereof—the
incubatory pouch—until they are far advanced in development; and usually
leave the body as tailed larvae. In many species, the ova and spermatozoa
mature at different times in the life-history, and so self-fertilisation is
prevented. Some species (such as many Botryllidae and Distomatidae) are
protogynous, the ova being produced and shed before the testes have
matured, while other species (_Coelocormus huxleyi_) are protandrous, being
male while young and female later. But there is no doubt that in other
cases (e.g. _Ascidia mentula_) self-fertilisation is not only possible, but
does take place. After maturation certain of the follicle-cells which
invest the ovum in the ovary migrate into the egg and proliferate so as to
form a layer in the superficial part of the egg, where they appear as the
so-called "testa-cells" or "kalymmocytes" (Fig. 25, A, _t.c_). The
remaining follicle-cells may form two or more layers, usually one of large
cubical cells, which may become greatly vacuolated, next to the ovum, and
an external flattened layer which is cast off when the egg escapes from the
ovary.
Segmentation is complete and results in the formation of a spherical
blastula with a small segmentation-cavity (Fig. 25, C). The blastula grows
larger and begins to differentiate.[96] There are slightly smaller cells
which divide more rapidly at one end of this embryo, the future ectoderm,
and slightly larger and more granular cells at the other, which become
chiefly endoderm (hypoblast). Invagination of the larger cells then takes
place (Fig. 25, D), resulting in the formation of a gastrula with an
archenteron. The hypoblast cells lining the archenteron become columnar
(_hy_). The curving and more rapid growth at the anterior end of the embryo
narrow the primitively wide open blastopore, and carry it to the posterior
end of the future dorsal surface (Fig. 25, E). The orientation of the body
is now clear.
{57}[Illustration: FIG. 25.—Embryology of Ascidian. A, mature ovum: _foll_,
follicle-cell; _m_, membrane; _n_, nucleus; _p_, protoplasm; _t.c_,
test-cell; B, mature spermatozoon; C, segmentation-stage in section to show
blastocoel; D, early gastrula-stage; E, later gastrula-stage; F, later
embryo showing rudiments of notochord and neural tube; G, transverse
section of body of embryo showing mesoblast and formation of neural canal;
H, late embryo showing body and tail, notochord, neural canal, and
mesenteron; I, young larva ready to be hatched; K, transverse section of
tail of larva. _ar_, Archenteron; _at_, atrial invagination; _au_, otocyst;
_b.c_, blastocoel; _b.p_, blastopore; _ch_, notochord; _ep_, epiblast; _f_,
tail-fin; _hy_, hypoblast; _m.b_, mesoblast; _mes_, mesenteron; _musc_,
muscle-cell; _n.c_, neural canal; _ne.c_, neurenteric canal; _n.v_, neural
vesicle; _oc_, ocellus. (Modified from Kowalevsky and others.)]
The embryo is elongated antero-posteriorly, the dorsal surface is
flattened, and the blastopore indicates its posterior end. Around the
blastopore the large ectoderm cells form a medullary plate, along which a
groove (the medullary groove), runs forwards, bounded at the sides by
medullary folds which meet behind the blastopore. Underneath the posterior
part of the medullary groove certain of the hypoblast cells from the dorsal
wall of the archenteron, in the median line, form a band extending forwards
(Fig. 25, E, _ch_). This band separates off from the hypoblast, which
closes in beneath it, and thus gives rise to the notochord (Fig. 25, F).
The more lateral and posterior cells become mesoblast, and separate off as
lateral plates, which show no trace of metameric segmentation (Fig. 25, G).
The remainder of the archenteron {58}becomes the branchial sac, and by
further growth buds off the rest of the alimentary canal.
The medullary groove now becomes converted into the closed neural canal by
the growing up and arching inwards (Fig. 25, G, _n.c_) of the medullary
folds, which unite with one another from behind forwards in such a way that
the blastopore now opens from the enteron into the floor of the neural
canal, forming the neurenteric passage (Fig. 25, F, _n.e.c_). For a time
the anterior end of the neural canal remains open as a neuropore. By this
time the posterior end is elongating to form a tail, and the embryo is
acquiring the tadpole-shape (Fig. 25, H) characteristic of the free larva.
The tail grows rapidly, curves round the body, and also undergoes torsion,
so that its dorsal surface comes to lie on the left side. It contains
ectoderm cells on its surface, notochordal cells (in single file) up the
centre (see Fig. 25, H, _ch_), a neural canal dorsally, and a row of
endoderm cells representing the enteron ventrally to the notochord. Later
on the mesoblast also is prolonged into the tail, where it forms a band of
striated muscle-cells at each side of the notochord. When the ectoderm
cells begin to secrete the cuticular test this forms two delicate
transparent longitudinal (dorsal and ventral) fins in the tail (Fig. 25, K,
_f_), and especially at its extremity where radial thickenings form striae
resembling fin-rays. The ectoderm on the anterior end of the body grows out
into three adhering papillae (Fig. 26, A).