tube of the larva is largely non-nervous and there are no seg- 

 mental fibers extending out to the muscle cells. Innervation 

 appears to be from the visceral ganglion next to the brain 

 with transmission from cell to cell. The visceral ganglion 

 disappears with metamorphosis. 



The tunicate is peculiar in that it lacks a coelom. In the 

 larva the mesoderm (see Chapter 7) is formed as bands from 

 the dorsolateral wall of the gut. The notochord arises from 

 the cells between these two bands. This manner of origin is 

 similar to that in Amphioxus but here the similarity ends. 

 The tail muscles and body mesenchyme produced by these 

 strands develop directly without passing through a pouch 

 stage. The only cavities in the body are the blood channels 

 or hemocoels. 



The pericardial cavity evaginates from the posterior floor 

 of the pharynx and the heart forms within this. Bilateral 

 evaginations of the pharynx give rise to the epicardial sacs. 

 These may remain paired and open into the pharynx, or 

 they may separate from the pharynx and fuse to form a single 

 cavity. The epicardium is viewed as having an excretory 

 function since other more obvious structures for this are 

 lacking. 



Hemichordata 



There are two main divisions of hemichordates, the En- 

 teropneusta and the Pterobranchia. Recently a third group 

 was added— Planctosphaera. Its sole genus appears to be only 

 a specialized tornerian larva, whose name stems from the 

 fact that these ciliated animals formerly were identified as 

 belonging to a genus Tornarm. They are now recognized as 

 a developmental stage in the life cycle of many species of 

 enteropneusts. A somewhat similar larva also occurs in the 

 pterobranch Cephalodiscus. 



Enferopneusts Balanoghssus (from the Greek words balanos 



meaning acorn and ghssa, tongue) is the generic name of 



some, and a common name of many, species belonging to a 



worm-like group of organisms identified in 1870 by Gegen- 



baur as the Enteropneusti (intestine breather). There are 



twelve genera in the group, including Saccoglossus (sack 



tongue), Glossobalanus, and Ptychodera (split neck). In size 



these animals are small, or at least slender; they range in 



length from 10 cm to as much as 2 m. They are bilaterally 



symmetrical. The body is ciliated and divided into three 



distinct anteroposterior divisions (Figure 1-10). The most 



anterior part is the protosome (or proboscis), which may be 



long and slender (Saccoglossus) or short and acorn-like, 



Pivchodera or Glandtceps. The second segment is the collar or 



mesosome. The mouth opens below the thin or tapered 



connection of the proboscis with the collar. Behind the 



collar is the metasome. On the dorsal surface of the meta- 



some are bilateral lines of small pharyngeal openings, which 



lie to either side of the slight ridge formed by the dorsal 



nerve cord. In some of the enteropneusts, genital ridges, or 



wings, extend up on either side and may meet above the 



dorsal body wall {Balanoghssus or Ptychodera). Posterior to 

 the pharyngeal openings the lateral body wall has many, 

 serially repeated, sacculations produced by the hepatic out- 

 pocketings of the gut. There are mid-dorsal and ventral 

 ridges, or grooves, marking the main nerve cords. The anus 

 is at the extreme posterior end of the body. 



The mouth or buccal tube extends through the collar into 

 the metasome where it becomes the pharynx (Figure 1-11). 

 The stomochord, or buccal diverticulum, is a simple, an- 

 teriorly projecting diverticulum of the buccal tube which 

 projects forward into the protocoel, the coelomic cavity of 

 the protosome. The pharynx opens to the exterior by way 

 of a large number of U-shaped slits which lead into sac-like 

 pouches opening to the surface through separate or common 

 pores. The pharynx is frequently divided into a dorsal "slit" 

 passage and a ventral "esophageal" passage by a fold from 

 either side (Figure 1-11 B). These folds do not meet at the 

 midline. Posterior to the pharyngeal region there may be a 

 small dorsal diverticulum in the genus Glossobalanus. 



BALANOGLOSSUS 

 NUMEENSIS 



-pharyngeal pore 



genital wing 



with genital pores 



Ir*-' 



mi 



genital folds 



hepatic caeca^ 



3b' 



./ 



SACCOGLOSSUS 

 PUSILLUS 



BALANOGLOSSUS 

 CLAVIGERUS 



Figure 1-10. External appearance of several enteropneusts. (After 

 Dawydoff, 1948) 



12 . CHORDATES, PROTOCHORDATES, AND THEIR RELATIONSHIPS 



