THE ANIMAL KINGDOM 



7 



The so-called nolo(ln)r(l is a diverticulum of the intestine which extends 

 with a narrow lumen into the proboscis from a point just behind the mouth. 



The larva of Balanoglossus, known as Tornaria, shows rather striking 

 resemblances to the larva of echinoderms. As in echinoderms, the 

 blastopore becomes the anus. The sub-phylum, therefore, is included in 

 the group of Deuterostomia. 



Cephalodiscus and Rhabdopleura are genera which show resemblances 

 to Balanoglossus but which have a U-shaped alimentary canal. Rhabdo- 

 pleura is without gill apertures. 



Sub-Phylum Urochorda (Tunicata) 



The urochordates, the tunicates or sea-squirts, are so named because 

 the notochord, absent in the sessile adult, is always limited to the tail 



TENTACUES 

 CIUATED GBOOVt 



SUBNEURAL GLAND, 



BRAIN 

 ^AMlFl, 



NERVE CORD 

 ESOPHAGEAL 



ASCIDIA-A UROCHORDATE 



Fig. 8. — Ascidia, a urochordate. The animal is viewed as if cut in median longitudinal 

 section and as seen from the right side. (Redrawn from Sewertzoff, after Boas.) 



region. Another character common to the group is the presence of a 

 tunicin mantle which is secreted by the skin. Tunicin is a chemical 

 substance that resembles cellulose. A coelom is sometimes present, but is 

 limited to the region of the ventral heart. Nephridia or coelomoducts are 

 wanting. The body is unsegmented, and the alimentary canal is bent on 

 itself so that the anus lies near the mouth. The pharynx is perforated 

 by gill-slits, the number of which varies greatly in the different species. 

 The nervous system consists of a nerve ganglion dorsal to the pharynx, 

 from which nerves extend to the various organs. In some forms both 

 sexual and asexual methods of reproduction occur. Individuals, however, 

 are usually hermaphroditic. Development generally involves metamor- 



