PHYLUM CHORDATA 



389 



egg develops into a free-swimming larva called a Tornaria (Fig. 

 334). When first discovered, these larvae were thought to belong 

 to an echinoderm. The resemblance of the Tornaria to the larvae 

 of echinoderms (Figs. 150-151) is quite striking and has led to 



FIG. 335. Rhabdo pleura. 

 a, mouth; b, anus; c, stalk; 

 d, proboscis; e, intestine; /", an- 

 terior region of trunk; g, a ten- 

 tacle. (From Parker and Has- 

 well, after Lankester.) 



FIG. 336. Cephalodiscus dodeca- 

 lophus, anterior view. i, tentacles; 

 2, proboscis (buccal shield); 3, pig- 

 ment band on proboscis; 4, buds; 

 5, pedicle; 6, trunk. (From Sedg- 

 wick's Zoology, after Mclntosh.) 



a rather plausible theory of the origin of the vertebrates (Chap. 

 XXII). 



Rhabdopleura (Fig. 335) and Cephalodiscus (Fig. 336) are 

 colonial ENTEROPNEUSTA inhabiting the deep sea. They have 

 the power of reproducing by means of buds (Fig. 336, 4). 

 Cephalodiscus has only one pair of gill-slits; Rhabdopleura has 

 none. 



2. SUBPHYLUM II. TUNICATA 



The TUNICATA or IJROCHORDA (Fig. 337) all live in the sea. 

 They are either free-swimming or attached, are widely distrib- 

 uted, and occur at all levels from near the surface to a depth of 



