THE PHYLA HEMICHORDATA AND CHORDATA 



lationships between the siphons, pharynx, esophagus, stomach, intestine, and 

 anus are apparent in Figure 18.3. A digestive gland is also present, connected 

 with the stomach by a duct. Water passes into the mouth, enters the bran- 

 chial sac, and passes through the many small openings in its wall into the 

 atrial cavity on either side, and thence to the excurrent siphon, from which 

 it is discharged. The openings in the branchial sac thus function in the 

 manner of the less numerous openings called gill slits in other chordates. As 

 in hemichords, food is obtained by straining the nutrient material from water 

 received through the mouth and discharged through lateral openings in the 

 pharynx. In the tunicate the food particles are caught in mucus as the water 

 leaves the pharynx and are conveyed by ciliary currents along a specialized 

 path to the esophagus; at the same time the oxygen dissolved in this water 

 diffuses into the blood within the vessels of the pharynx. 



The circulatory system consists of a tubular heart, lying along the outer 

 curvature of the stomach, with tubules extending from one end directly to the 

 pharvnx and mantle, and from the other end to the stomach and nearby 

 organs and thence to the pharynx. There are no true blood vessels, the 

 blood circulating through extensive tubular cavities which lack an endothelial 

 lining. The blood contains several kinds of free cells, some of which contain 

 pigments which function in the transport of oxygen. A unique feature 

 of the circulatory system is the periodic reversal of the heart beat; after the 

 peristaltic waves of contraction have swept across the heart in one direction 

 for a time, they cease, shortly to be resumed in the reverse direction. This 

 brings about a corresponding reversal in the flow of the blood. 



In the adult ascidian there is no cavity that can be called a coelom, unless 

 the pericardium and the cavities within the excretory and reproductive organs 

 can be so designated. Excretory functions are assumed for a mass of cells, 

 without a duct, lying near the intestine; it has been shown to contain uric 

 acid. 



The nervous system consists of a single elongated ganglion, embedded in the 

 mantle between the two siphons, and of sensory and motor nerves extending 

 from each end of the ganglion to the siphons and other parts of the body. A 

 glandular mass beneath the ganglion has been compared to the hypophysis of 

 vertebrates. 



Most tunicates are monoecious. The reproductive system includes an ovary 

 and a testis, lying against the intestine, with ducts opening into the atrial 

 cavity near the anus. In most of the .solitary ascidians, such as Molgula, the 

 gametes are discharged into the atrium, and fertilization occurs in this cavity 

 or in the external water, where development takes place. The stages of 

 cleavage and early differentiation are comparable with those of Branchiostoma 

 (see Fig. 5.14). Embryonic development culminates in the appearance of a 

 larva, the so-called tadpole, which possesses a dorsal, tubular nerve cord, 

 a notochord, and gill slits (Fig. 18.4). Later, this larva becomes attached by 

 suckers at its anterior end and undergoes a complicated metamorphosis, dur- 

 ing which its more conspicuous chordate characteristics are lost or modified. 



547 



