THE PHYLA HEMICHORDATA AND CHORDATA 



from which by successive radiations the ancestors of the modern enterocoelous 

 groups arose. The hemichords would seem to have evolved from this an- 

 cestral line after the divergence of the ancient echinoderms and chaetognaths 

 but before the rise of the true chordates. These evolutionary changes must 

 have antedated the beginning of our fossil record and undoubtedly involved 

 small forms lacking hard parts. 



The Phylum Chordata 



THE ACRANIATA 



The Urochordata. These organisms constitute one of the three sub- 

 phyla into which the phylum Chordata is divided. They are invertebrates, 

 in that they lack vertebrae, but their characteristics are unmistakably those 

 of the chordates. The subphylum Urochordata includes the classes Ascidiacea, 

 Lorvaceo, and Thaliacea, all ol which arc marine animals. Of these, the 

 Ascidiacea, or sea squirts, are the best known; the Larvacea are an unusual 

 t\pe; and the Fhaliacea are the most highl\- specialized. 



The Ascidiacea. The important features of the ascidians are the specializa- 

 tions of the adult for an attached mode of life and the apparent evolution of 

 the group from free-moving ancestors. On the basis of superficial and func- 

 tional characteristics, ascidians were long classified as moUusks; it was not 

 until their life cycles became known, in 1866, that the chordate nature of the 

 early developmental stages was recognized. This establishment of relationship 

 through the study of developmental stages is a celebrated event in the history 

 of embryology. 



The sea squirt, Molgu/a manhottensis, is one of the species of tunicates most 

 abundant along the Atlantic coast of North America. Molgu/a is found at- 

 tached to various submerged objects in shallow water, but it is most easily 

 collected from the piling under wharves, where it is commonly associated with 

 such other ascidians as Styela and Cwna (Fig. 18.3). An expanded Molgula 

 appears as a globular mass attached at one end and having two tubular 

 processes, the siphons, extending from the other. If an undisturbed specimen 

 is observed, a gentle current of water may be seen entering the longer of these 

 siphons and flowing outward from the shorter one. As in pelecvpod mollusks 

 (p. 372), the entire economy of the ascidian depends on these currents of 

 water, which, again as in the mollusk, are maintained by internal ciliary 

 activity. From the stream entering the incurrent siphon, the animal obtains 

 its food by straining out minute organisms and particles of detritus; and the 

 blood is aerated as the water passes through the modified gill slits. The feces 

 and reproductive products are carried outward by the water as it flows 

 through the atrium and the excurrent siphon. 



In any typical ascidian the entire body is covered by a membrane, thick 

 in Molgula and Styela but thinner in Ciona, termed the test or tunic. This 



545 



