212 CHORD ATA 



the molluscs. Later, they were associated with the worms. Their 

 development, however, shows them to be related to the Vertebrates. 

 The larval ascidian is more highly developed than the adult. 



Anatomy and Physiology. — The tunic or mantle has the same 

 chemical constituents as cellulose (CeHioOs). Nowhere among 

 animals is there such a rich formation of cellulose. The anterior 

 part of the digestive tract is modified into a pharynx or branchial 

 chamber, the walls of which are perforated by a number of gill 

 slits leading to the exterior, or to a peri-branchial chamber, and from 

 this to a cloaca. While respiratory water passes through the gill 

 slits, the food particles which it contains are received by a ring- 

 shaped ciliated band and, enveloped by mucus, are led to the esoph- 

 agus. The mucus is formed by a ciliated glandular groove, the 

 endostyle on the ventral surface of the pharynx. The ventral tubular 

 heart lies in the pericardium between the gill region and the stomach. 

 It changes the direction of its contractions frequently, first driving 

 the blood to the gills, then resting; then pumping it from the gills 

 and sending it to the stomach. The ductless excretory organs are 

 in the second loop of the intestine; the dorsal ganglion and the 

 subneural gland are the only remains of the comparatively well- 

 developed brain of the larva. The subneural gland which is asso- 

 ciated with the dorsal ganglion is homologous to the hypophysis ^ 

 of the higher vertebrates. The gonad is hermaphroditic, the testis 

 surrounding the ovary, and the animal is first a female, then after 

 pregnancy becomes a male. The larva of the ascidians is active, 

 swims by a long tail, looks like a tadpole, and has a notochord. 

 (Figure loo.) 



Description of the Larval Ascidian. — The tail is fringed with a 

 caudal fin which is an outgrowth of the thin test covering the whole 

 surface. The notochord is in the axis of the tail. The nerve cord is 

 dorsal and forms the trunk ganglion and sense vesicle with otocyst 

 and eye. The digestive system consists of pharynx, esophagus, 

 stomach and intestine. The larva remains only a few hours in the 

 tailed free swimming stage. Then It becomes fixed by adhesive 

 papillae and begins to undergo retrogressive metamorphosis. 

 Molgula manhattensis (which has no tailed form) reaches its full size 



^ Butcher (1930), Jour. Exp. Z06I., vol. 57, no. i, pp. i-ii, has shown that in 

 Molgula manhattensis the pituitary gland has the oxytocic principle of posterior pitui- 

 tary. (See page 448.) 



