Lancelets: A New Look at Some Old Beasts, The

The Lancelets: A New Look at Some Old Beasts. CARL GANS, NORMAN KEMP, AND STUART POSS, eds. Israel Journal of Zoology Vol. 42, Supplement. Laser Pages Publishing Ltd., Jerusalem, Israel, 1996, 446 pp. Extensive Bibliography, Subject index. Paper ISSN 0021-2210

It started as a new mollusc species, the slug Limax lanceolatus. Later it became a new genus of fish, Branchiostoma ("gill-mouth"), so called because the author believed that the oral cirri were external gills. Sometimes it was given the generic name Amphioxus ("pointed at both ends"), indicating that this animal is devoid of a head end. It is now known as Branchiostoma, even if it has no gills, just pharyngeal slits. But it has a head end and a brain with some characteristic vertebrate features; the species is a peculiar acraniate vertebrate without vertebrae.

"The Lancelets" is based on reports from a workshop on cephalochordates. It contains 16 chapters written by 26 experts in a range of acranian fields. The book has been carefully edited and completed. As a result, it is not the usual type of heterogeneous, congress-proceedings publication. Many authors provide more background information than is common in most primary reports, and results are discussed in a format that also is accessible to nonspecialists. This is especially welcome in a young and explosive research field, such as lancelet molecular biology, which is elegantly presented by P W. H. Holland.

The contribution of Poss and Boschung represents a much needed clarification of the complicated taxonomy, systematics and biogeography of lancelets. There are about 29 valid acranian taxa with two extant genera, Branchiostoma and Epigonichthys (formerly Asymmetron). Most likely, this particular chapter will be a standard reference work for many years to come.

The lancelet larva feeds while hovering in an oblique position. Gilmour studied the complicated feeding mechanism with scanning electron microscopy and in vivo with video recordings. An intricate mucus trapping device uses secretions from the endostyle and club-shaped gland to capture particles between the mouth and the first pharyngeal slit and carry them backwards into the intestinal canal. The pronounced larval asymmetry is an adaptation for this very specialized feeding mechanism (Presley, Harder and Slipha). Topographic similarities of specialized structures during development therefore may be unreliable for phylogenetic discussions. Instead, these should be based on developmental processes, such as germ layers and metameric development, that are similar to those of craniates.

Skillful electron microscopic work and impressive reconstructions have made it possible for Lacalli to describe new structures in the anterior part of the larval nerve cord. They are compared with probable corresponding forms and configurations in the craniate brain. One could question, however, whether there is sufficient evidence to adopt terms such as "tectum" and "balance organ" for the lancelet brain. Boeke's erroneous description in 1902 of an infundibular region in the lancelet brain can still cause some confusion.

The lancelet renal system is intermediate between a proto- and a metanephridial system, and it can be understood in terms of vertebrate nephron structure (Ruppert). The Hatschek's nephridium is asymmetrical because its branchial partner was the first modified branchial cleft, which is now the mouth. When the mouth rotates during metamorphosis from the left body side to become a symmetrical adult velum, Hatschek's nephridium remains unpaired in the asymmetrical right position.

Other contributions on Branchiostoma species deal with muscle development and biochemistry (L. Z. Holland), and one, single article provides an ecological study: the population dynamics of the Florida lancelet (Stokes and N. D. Holland). But where are the Epigonichthys lancelets? Are they totally forgotten by today's researchers?

The book is attractively produced, even if some illustrations are rather small and dark with labels that are hard to read. In one case a lancelet larva is presented upside down, but it does not really matter as this illustration adds nothing to the text.

Carl Gans' brief introductory review is a fascinating survey of the field of "acraniology" in all aspects. Gans and coauthors have performed an outstanding service in bringing together so much information for scientists interested in acranians and related topics. The book's extensive bibliography alone, which contains some 2,700 references, is enough to make "The Lancelets" a must for those working on protochordate. It is also essential to anyone interested in lower deuterostomes.

RAGNAR OLSSON

Stockholm University Functional Zoomorphology S-106 91 Stockholm, Sweden

Copyright Society for Integrative and Comparative Biology Sep 1997
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