base of the arms to below the mouth. The whole lopho- 

 phore is ciliated and food is secured through the secretion 

 of its glandular cells (fibrous mucous) and the action of 

 the cilia. 



The digestive tract begins with a buccal (mouth) tube in 

 the collar region, from which a stomochord is evaginated 

 anterodorsally. This lies in contact with the septum, between 

 the protosome and collar, and in contact with the dorsal 

 epidermis of the collar region in which lies the brain. The 

 lumen of the stomochord may remain open or be reduced 

 to a series of spaces. Behind the stomochord there may be 

 a dorsal recess of the buccal cavity. The pharynx is short 

 and lined with ciliated epithelium. There is a single pair of 

 slits in Cephalodiscus and Aluhana. but there are none in 

 Rhabdopleura. Associated with the undivided slit is a dorso- 

 lateral pharyngeal sac. The esophagus is short and leads into 

 the large sac-like stomach; this in turn leads into the in- 

 testine, which curves anteriorly and dorsally to open at an 

 anus just behind the collar. The stomach and intestine are 

 not lined with cilia. 



The coelom of the protosome is a single space opening to 

 the exterior by a pair of canals and pores. The mesosome 

 and metasome have bilateral spaces; that of the mesosome 

 opens to the exterior by paired pores behind the oral lamella 

 and just in front of the pharyngeal slits. It extends upward 

 into the arms of the lophophore. These body spaces are not 

 conspicuously lined, for muscle fibers and loose mesenchyme 

 lie within them. 



The circulatory system consists of a dorsal sinus (unlined), 

 which extends forward along the esophagus and pharynx to 

 terminate in a central sinus (heart) on the tip of the stomo- 

 chord. The heart and its surrounding pericardium project 

 into the protocoel. The pericardium is muscular and con- 

 tractile as in the enteropneust. Blood leaves the glomerulus- 

 like central sinus (there is no apparent glomerulus in terms 

 of a glandular and folded coelomic lining) through a channel 

 ventral to the stomochord, which divides to bypass the mouth. 

 A ventral vessel extends back along the body wall and 

 around the posterior end of the stalk to the dorsal side. Here 

 it divides into an intestinal and a dorsal body wall vessel. 

 A plexus enclosing the gonads opens into the dorsal sinus. 

 Cells occur in the blood. 



The nervous system consists of an epidermal plexus of 

 fibers and cell bodies, in which thickened strands occur. The 

 collar ganglion is the main concentration and lies dorsally. 

 From it a midline and bilateral protosome bands extend 

 forward. Behind it there is a mid-dorsal strand extending to 

 the anus, and there are also bilateral circumenteric bands 

 which pass in front of the pharyngeal slits to the underside 

 of the body where they join the ventral midline cord. One 

 or two lateral cords connect with this circumenteric band. 

 The ventral midline trunk, like the blood vessel, extends 

 around the end of the stalk to the dorsal surface. 



Reproduction involves both gametes (sexual ) and budding. 

 The gonads are bilateral or unilateral (right side in Rhabdo- 



pleura), opening behind and lateral to the anus. The develop- 

 ment of the bud is direct. The buds arise from the tip of the 

 stalk. 



One of the most interesting aspects of the hemichordates is 

 their larval development. Some kinds of enteropneusts have 

 a ciliated larva, the tornaria, while others and the ptero- 

 branchs develop directly. Both types of development may be 

 observed within a single genus, Balanoglossus. The early 

 stages of both types of development are similar and there is 

 much agieement even in the later stages. 



In the development of the larva (see Chapter 7), three 

 pairs of mesodermal pouches evaginate from the archenteron 

 (enterocoelous development). These pouches show much 

 variation in their exact mode of origin but in any case agree in 

 most details with the three pairs of pouches observed in the 

 larva of some echinoderms. The resemblance between the 

 tornaria larva and the auricularia or bipinnaria types of lar- 

 vae of echinoderms has been used to suggest a relationship be- 

 tween these phyla and the chordates. 



General observations 



In terms of their pharyngeal structure, epidermal nervous 

 system with both dorsal and ventral cords, body muscula- 

 ture, tripartite body, and lack of a notochord (or presence 

 of a stomochord), the hemichordates differ from the tuni- 

 cates and Amphioxus. Recognition of the hemidiordates as 

 a separate phylum seems reasonable; yet one must assume 

 some kind of relationship to the chordates, perhaps best ex- 

 pressed as common ancestry. If these phyla had a common 

 stem, then the hemichordates would represent an offshoot 

 developed before the appearance of the notochord. 



Of the protochordates, Amphioxus seems most like the 

 vertebrate, but the tunicate also shows the multiplication of 

 coelomic and myotomic units, which these organisms share 

 with the vertebrate. To give some meaning to this last 

 statement, the hypothetical primitive vertebrate should be 

 characterized, utilizing some of the anatomical information 

 gained from the description of the protochordates. Such a 

 characterization will necessarily anticipate some of the con- 

 clusions regarding the vertebrates. -As such it is a precon- 

 ception and should be so treated. It is hoped that the student 

 will reconsider the problem of an ancestral form, and its 

 relationships to the protochordates, after a detailed study of 

 the vertebrates. 



As a generality, the ancestral vertebrate was more com- 

 plex in every detail of its structure than the invertebrate. 

 The body was covered by a stratified, not simple, epidermis 

 and was divided into a head with branchial pharynx, trunk, 

 and tail. There were segmental myotomes extending through 

 these body divisions. There were at least three myotomes in 

 the head which involved the protosome, mesosome, and a 

 part of the metasome of the ancestral form. To these initial 

 segments new ones were added posteriorly. The caudal 



16 • CHORDATES, PROTOCHORDATES, AND THEIR RELATIONSHIPS 



