I/. TUNIC AT A. 



505 



SUB PHYLUM II. TUNICATA (UROCHORDA). 



In their adult condition the Tunicata, or sea-squirts, bear some 

 resemblance to the siphonate mollusca, especially in the posses- 

 sion of incurrent and excurrent orifices, usually close together, 

 and a mantle. Hence these forms were long associated with the 



FIG. 543. Diagram of a tunicate (orig.). a, atrium ; 6, nervous ganglion ; e, endo- 

 style ; i, intestine; m, mouth; n, subneural gland; s, stomach; /, tunic. In the 

 centre the branchial basket with the gill slits communicating with the peri- 

 branchial space, and this in turn with the atrium. 



molluscs ; later they were associated with the worms, but their de- 

 velopment shows them to be more nearly related to the vertebrates. 

 The group owes its name to the tunic or mantle lacking in the 

 Copelatse an envelope (fig. 543, t) which is formed like a cuticle 

 by the epithelium of the skin, but which is distinguished from 

 ordinary cuticula by its structure. It much resembles connective 

 tissue in that cells from the mesoderm wander into the ground 

 substance, which is sometimes fibrous, sometimes homogeneous, 

 and has an interesting chemical nature. It consists of the same 

 proportions of carbon, oxygen, and hydrogen (C 6 H 10 6 ) as cellulose 

 and agrees with this substance, so characteristic of plants, in its 

 reactions (blue color with iodine-iodide of potassium and sulphuric 



