An Introduction to the Invertebrates, Second Edition - tiera.ru

WHAT ARE THE HEMICHORDATES?241the vertebrate specialisations. Genetic evidence confirms the primitivenature of the amphioxus but questions the closeness of therelationship (see Chapter 20).Urochordates have been very hard to place. A classic early accountby W. Garstang, followed by N. J. Berrill, invoked ‘neoteny’, wherethe larval form becomes sexually mature; if a tadpole larva were toreproduce, the result would be an animal very like the amphioxus(or at least, very like its larva before adult specialisations develop).The sessile and gelatinous tunicate adult would then be entirelyomitted from the life cycle. Species showing both kinds of life cycle(with an adult in the sessile tunicate or the chordate form) surviveto the present day, placed in different phyla.The phenomenon of ‘paedomorphosis’ (reproduction in the larvalform) is known from a number of present-day animals. It is called‘progenesis’ when gonadal development is accelerated and reproductionoccurs earlier in the life cycle, or ‘neoteny’ when somaticdevelopment of adult tissues is delayed and reproduction occursat the time normal to that species but while it is still a larva.Paedomorphosis is one way in which the normal process of naturalselection can result in sudden abrupt changes in form. It might haveoccurred several times in the chordate lineages: larvaceans such asOikopleura might be products of neoteny within the tunicates, andthe vertebrates resemble the larva rather than the specialised adult.Chapter 20 discusses the role of neoteny.Because of the great interest in the origin of vertebrates, the fullgenome of the ascidian Ciona intestinalis has recently been sequenced(see Chapter 20), but it is not possible to trace the course of chordateevolution with certainty. What is certain is that the chordatecharacters, at whatever point they arose, proved to be excellentadaptations to a free-swimming life in the sea.HEMICHORDATA18.4 What are the hemichordates?They consist of two groups, very dissimilar in appearance.18.4.1 EnteropneustaThe enteropneusts (70 species, 9 200 cm in length) are large solitaryworm-like animals such as Balanoglossus, burrowing in the mud orsand of the shallow seas. The body consists of three regions,a proboscis, collar and trunk (Figure 18.4a,b), corresponding to thethree divisions of the embryonic coelom, the protocoel (proboscis),the mesocoel (collar) and the metacoel (trunk). The ciliated probosciscollects food into the mouth, which opens from the collar. The lobein front of the mouth has a complex ‘heart/glomerulus’ system,