386 ''"^ ANIMAL KINGDOM 



found in animals, is present in the tunic. The animal is attached to 

 the substrate by its base, and two tubular openings are present near the 

 upper surface. The uppermost one, or incurrent siphon, leads into a 

 large, barrel-shaped pharynx, which occupies most of the space within 

 the body. The gill slits in the pharyngeal wall do not open directly to 

 the body surface, but into a specialized, ectodermally lined chamber, 

 called the atrium, lying on each side of the pharynx and along its dorsal 

 edge. The atrium opens at the surface through an excurrent siphon. 

 Ciliated cells in the pharynx maintain a flow of water into the incur- 

 rent and out of the excurrent siphon. 



Gas exchange occurs between the water passing through the 

 pharynx and blood channels in the pharyngeal wall, but the pharynx 

 is also a food-gathering mechanism. Mucus produced in the endostyle 

 (a longitudinal groove in the floor of the pharynx) is moved across the 

 lateral walls of the pharynx to its dorsal surface. Minute food particles 

 are entrapped in this sheet of mucus, which is then carried along a 

 dorsal band into the more posterior parts of the digestive tract. The 

 intestine finally opens into the atrium. 



A tube-shaped, muscular heart is enclosed in a reduced coelom, and 

 a vessel leads out from each of its ends into open channels in the wall 

 of the pharynx and other organs. Capillaries are absent. The beating 

 of the heart is unique in that waves of contraction move from one end 

 of the heart to the other for a while, and then the beat reverses and 

 the contractions move in the opposite direction. The heart and blood 

 vessels have no valves. 



A solid nerve ganglion, from which nerves extend to various parts 

 of the body, lies in the mantle between the siphons, and a peculiar 

 neural gland lies beside the ganglion. The latter opens into the pharynx 

 by means of a short ciliated duct. Its function is uncertain, but some 

 investigators consider it to be an endocrine gland and have compared 

 it to the pituitary gland of vertebrates. 



Ascidians are hermaphroditic, part of the gonad being ovary and 

 part testis. One or more ducts lead from the gonad to the atrium. Cer- 

 tain ascidians are self-fertilizing, that is, the eggs of one individual can 

 be fertilized by sperm from the same individual, but in others the 

 sperm must come from a different individual. Asexual reproduction by 

 budding also occurs. 



Pharyngeal gill slits are well developed in the adult, but one must 

 examine a tunicate larva to find the other chordate characteristics (Fig. 

 20.2, B). The larva is tadpole-shaped with an expanded body and a 

 long mobile tail equipped with longitudinal muscle fibers. A notochord 

 supports the tail (whence the term urochordate) and a distinct tubular 

 nerve cord lies dorsal to it. The anterior end of the nerve cord expands 

 to form a brainlike sensory vesicle containing a light-sensitive ocellus 

 and an otolith concerned with equilibrium. The pharynx and other 

 digestive organs develop within the body, but do not function in most 

 larvae. A pair of dorsal, ectodermal invaginations, which eventually ac- 

 quire a common external opening, grow down beside the pharynx to 

 form the atrium. Within a day or two the tadpole finds a favorable 



