The science of life : fully illustrated in tone and line and including many diagrams

THE FIRST GREAT PHYLUM: VERTEBRATES

common gelatinous mass. Some are not sessile but, beautifully transparent, float about in the sea ; of these the most striking is Pyrosoma, whose thimble-shaped colonies—up to twelve feet in length—are among the most brilliantly phosphorescent objects known. A few never discard the tail and notochord of their youth but spend their lives as active, tadpole-like creatures, inhabiting relatively large gelatinous houses that they secrete and propel through the open waters of the sea.

Balanoglossus is another extraordinary associate of our phylum. It is a white or orange worm-like creature—the length varying in different species from an inch to three yards —that spends its life burrowing in mud at the bottom of shallow seas. It is far less like a vertebrate than either Amphioxus or a young sea-squirt, yet it is more like a vertebrate than it is like the members of any other phylum. It has, for example, a great number of gill-slits leading out of the sides of its throat, and the possession of a perforated throat is apparently a chordate peculiarity ; as the animal nourishes itself by swallowing mud or sand and digesting out any organic matter as it passes through the intestine, it may be presumed that the slits are respiratory. Again, the front part of the body is very mobile and is used for

burrowing ; it is supported by a thickwalled tube of narrow bore that runs forwards from the roof of the mouth. The cells of this tube have a structure suggestive of the cells of a notochord, and it lies, like a notochord, just below the principal nerve-chord ; moreover, in embryo vertebrates the notochord develops as a tube that is pinched off from the roof of the digestive tube. It is at least possible, therefore, that this structure is homologous with the notochord. On the other hand Balanoglossus has a point of resemblance with a very different phylum, the echinoderms (starfishes, sea-urchins, and their allies) ; in many species the eggs hatch into minute larve that are like the larve of sea-urchins, except that they grow into Balanoglossus. Thus to link vertebrate and echinoderm is a remarkable achievement.

Classified in the same group as Balanoglossus are two genera of similar creatures that live like polyps in colonies surrounded by gelatinous tubes, and capturing suspended particles by means of their branching tentacles. The best known is called Rhabdopleura. It marks the extreme limit of the chordate phylum. The chordate ground plan, reduced to the mere shadows of backbone, liver, and gill-slit, is traceable no further among living things.

Fig. 77. Semi-vertebrates : on the left, two Sea-Squirts ; above, two Lancelets, of which one is buried in sand with only its mouth above the surface ; on the right, a Balanoglossus.

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