GENERAL ZOOLOGY 



colonies of various kinds. In Botryllus, for example (Fig. 18.5), colonies 

 consist of several individuals arranged radially about a common excurrent 

 opening; each individual bears its own incurrent siphon. In Perophora, root- 

 like extensions, or stolons, grow over a surface and give rise to additional 

 individuals by budding. In most of the compound ascidians, the eggs are 

 fertilized and development proceeds within the atrium, as far as the tadpole 

 stage; the larvae are then discharged through the common excurrent atrial 

 opening. 



The Larvacea and Thaliacea. The Larvacea, which include only a few 

 species of peculiar structure, resemble somewhat the larvae of the ascidians, 

 rather than the adults. Whether they represent primitive, free-swimming 

 forms or persistent larvae of forms which no longer undergo metamorphosis 

 into sessile adults, is problematical. The Thaliacea include a wide range 

 of forms, mostly unattached and with some feeble powers of locomotion but 

 usually borne along by the currents of the ocean. They are clearly tunicates, 

 although highly modified in comparison with the ascidians. Their anatomy 

 and development indicate that they have evolved from ancestors which were 

 attached like the ascidians. If this is true, their ancestry must have 

 included primitive unattached types, then sessile forms, and finally second- 

 arilv free-moving organisms. Each of these phases has impressed its peculiar 

 adaptive modifications on the animals as we know them. In some Thaliacea 

 budding is extensively developed, and asexual generations alternate with 

 sexual ones. Development in some species occurs within the parent, the 

 embryo being attached by a placenta-like structure and nourished by difTusion 

 of nutrients from the parent's blood. Familiar thaliacean genera are Salpa 

 and Doliolum. 



The Cephalochordata. The subphylum Cephalochordata contains only 

 the class Amphioxi, a small group of marine animals long regarded as closely 

 related to the Vertebrata. The indications are, however, that their affinities 

 are with the tunicates, rather than with the vertebrates. It is suggested that 

 the ancestors of the modern Amphioxi diverged from tunicate-like forms, 

 giving up the attached mode of life to become free-swimming animals, and 

 giving rise in the long course of evolution to the group which we term the 

 cephalochordates. 



The Amphioxi: Branchio stoma lanceolatum. Among the few genera that 

 compose the class Amphioxi, the best-known species is Branchiosloma 

 { = Amphioxus) lanceolatum, which occurs in European waters. The common 

 name for this species is "lancelet," but the former generic name "Amphioxus" 

 has come to be the usual designation. The animals are found in shallow 

 water, buried in the bottom with the anterior end exposed, at rest on the 

 bottom, or swimming freely. The body of the amphioxus (Fig. 18.6) is 

 elongated and laterally compressed. A median dorsal fin, extending the 

 length of the body, and a shorter ventral fin are specialized posteriorly 

 to form a more conspicuous tail fin. Anterior to the ventral fin, two keel-like 

 ridges extend along the ventrolateral margins, marking the positions of the 



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