XII 



PHYLUM CHORDATA 



313 



of which is considered prse-oral, and the other post-oral, and an inde- 

 pendent circlet of strong cilia at the posterior end. 



Usually associated with Balanoglossus are two aberrant animals 

 Cephalodiscus and Rhabdopleura formerly regarded as Polyzoa. These 

 both resemble Balanoglossus in having the body divided into three parts 

 or regions a proboscis, with a proboscis cavity, a collar , with a collar- 

 cavity communicating with the exterior by a pair of collar-pores, and a 

 trunk with two distinct lateral cavities ; and in the presence of a 

 structure resembling a notochord with the same relations to the nervous 

 system as in Balanoglossus. They both differ from Balanoglossus in 

 having the alimentary canal bent on itself, so that the anal opening 

 is situated not far from the mouth ; in the presence of tentacles 

 arising from the collar ; and in the comparatively small size of the 

 proboscis. 



2. THE TJROCHORDA 



Still more unlike a Vertebrate in general appearance than Balano- 

 glossus, and yet, as the earlier stages show, 

 indubitably to be assigned to the Chordate 

 phylum, are the AscidiailS or Sea-Squirts 

 and their allies. 



Sea-squirts are familiar objects on rocky 

 sea-shores, where they occur often in large 

 associations, adhering firmly to the surface 

 of the rock. When touched the Ascidian 

 ejects with considerable force two fine jets 

 of sea-water, which are found to proceed 

 from two apertures on its upper end. The 

 shape of the Ascidian, however, can only be 

 profitably studied in the case of specimens 

 that are completely immersed in the sea- 

 water, specimens not so immersed always 

 undergoing contraction. In an uncontracted 

 specimen (Fig. 184) the general shape is 

 that of a short cylinder with a broad base 

 by which it is fixed to the rock. The free 

 end presents a large rounded aperture, and Fu;> l84 _ Ascidia, entire 



Some little distance from it on one side is animal seen from the right- 



hand side. (After Herd- 

 a second aperture of similar character. man.) 



