394 Comparative Morphology of Chordates 



Hemichorda (inaccurate, for the animal has much less than "half" a 

 notochord). The second subphylum includes animals most of which, 

 in the adult stage, are sessile, saclike things, unlike anything else, 

 vertebrate or invertebrate. These are the "sea squirts" or ascidians 

 (Fig. 308). The chordate features appear in the embryo, which develops 

 into a long-tailed larva. The notochord is confined to the posterior part 

 of the body and tail — hence the name of the subphylum, Urochorda, 

 The third subphylum contains the externally fishlike little "lancelet" 

 or Amphioxus (Fig. 312). The notochord extends not only throughout 

 the tail and trunk but almost to the tip of the head — therefore Cepha- 

 lochorda as the name of the subphylum. These three subphyla are 

 conveniently referred to collectively as the Protochordata, as set off 

 from the fourth subphylum, Vertebrata. The general plan of the 

 Phylum Chordata is, therefore, as follows: 



Phylum Chordata 



{Subphylum Hemichorda: Balanoqlossus 

 Subphylum Urochorda: Tunicates, including the saclike 

 ascidians and the hee-swimnnng tunicates. 

 Subphylum Cephalochorda: Amphioxus 

 Subphylum Vertebrata 



Nearly 50,000 species of chordates are known. 



Brief descriptions of the three protochordate subphyla follow. 



Subphylum Hemichorda 



The hemichordates hold a doubtful place in the animal kingdom. 

 It is not certain that their closest affinities are with chordates. The 

 adults are, in general, wormlike, but the larval stage closely resembles 

 the larva of an echinoderm. Their inclusion among chordates rests on 

 their possession of pharyngeal gill-slits, a short notochord-like out- 

 growth of the foregut, and a dorsal nerve-cord — but there is also a ven- 

 tral nerve-cord. There are possibly 50 species, all marine. 



Balanoglossus (Fig. 303) is the best-known representative. The 

 body is wormlike and is divided into three regions, proboscis, "collar," 

 and trunk. The proboscis is a hollow muscular organ with an opening 

 on the left dorsal side of its neck (Fig. 304). The mouth is on the ventral 

 side between proboscis and collar. The collar, like the proboscis, is 



COLLAR 



MOUTH GILLS 



Fig. 303. Balanoalossus, a hemichordate. (After Bateson. Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia. The Blakiston Company.) 



