coat made of tunicin, a material which resembles, or is, 

 cellulose. The tunic and tissues are nearly transparent. Free 

 cells occur in the tunic. The epidermis underlying the tunic 

 is simple, that is, formed of a single layer of cells which are 

 cuboidal or squamous (flattened) in shape. 



The tunicate shows certain similarities to Amphioxus in 

 that it has a complex pharyngeal structure with a ciliary- 

 mucoid feeding action. The pharynx is enclosed by an 

 atrium opening to the exterior through an atrial pore (Fig- 

 ure 1-8). The details of structure of the pharynx and atrium 

 of the two groups are close. The pharyngeal slits of the 

 tunicate are subdivided by connecting bars called synapti- 

 culae but not by tongue bars. The tongue bar, as observed 

 in Amphioxus, or the hemichordate, develops as a down- 

 growth from the dorsal wall of the slit which divides the slit 

 into anterior and posterior openings (Figure 1-12). There is 

 an expanded stomach and a simple intestine opening at the 

 anus into the atrium. The walls of the gut are not ciliated. 

 There is a pyloric gland formed of numerous ampullae 

 (small sacs) in contact with the intestine. Tubules from the 

 ampullae join to form a duct emptying into the digestive 

 tract just below the stomach. 



There is no coelomic cavity. The mesoderm consists of the 

 cells of the tunic, which wander out through the epidermis, 

 the circular and longitudinal muscles of the subepidermal 

 layer, mesenchyme cells of the interior, and the tissues of the 

 circulatory and reproductive systems. 



The circulatory system consists of a heart enclosed in a 

 pericardial cavity and connecting channels and sinuses. The 

 direction of blood flow through the heart may be reversed. 

 The blood contains cells but not erythrocytes (red blood 

 cells). The pharyngeal blood vessels are roughly comparable 

 to those of Amphioxus. 



Tunicates, usually hermaphroditic, have ovaries and 

 testes opening into the atrium through long ducts. Eggs and 

 sperm appear at different times in most species. Asexual 

 reproduction by means of budding is common. 



The brain is a solid ganglion between the mouth and the 



atrial pore, from which several nerves radiate. Associated 

 with the brain is a neural gland which has a ciliated funnel 



opening into the atrium. 



Embryological development It is in the larva of the tunicate 

 that there is a real resemblance to the vertebrate (Figure 

 1-9). The tadpole of Ctavetina has an ovoid body with a 

 long, slim tail attached. The tadpole swims actively for a 

 short period, settles to the bottom, and there metamorphoses 

 to the adult stage. 



In the body of the tadpole several structures are observed 

 which are asymmetrically placed. The beginnings of a 

 pharynx with a distinct endostyle, and subdivided pharyn- 

 geal sHts can be seen. The pharyngeal slits open into bilateral 

 atria, whose outer openings join to form a single external 

 atrial pore. The mouth is dorsal as is the atrial pore. The 

 gut ends blindly behind the atrial sac. The heart and 

 its pericardium arise from the pharynx and come to lie be- 

 hind the endostyle. A small epicardial diverticulum passes 

 back from the pharyngeal cavity to either side of the heart. 



In the tail is the notochord, made up of a line of vacuo- 

 lated ceUs. In many tunicates the number of notochord 

 cells is a constant (40 to 42). The notochord has a thin 

 fibrous sheath and is enclosed in three or four bilateral rows 

 of segmental muscle cells. These are elongate, fusiform, 

 smooth units with myofibrils around the periphery and a 

 central nucleus. The muscle cells overlap adjacent cells of 

 the notochord. 



The brain is an expanded vesicle in which there is a 

 ventral unicellular otolith (or statocyst) set on a patch of 

 sensory cells; on the right side is a simple eye formed of 

 several photosensory cells associated with a few lens cells. 

 These sensory organs function in the swimming activity of 

 the larva and guide it to the place of attachment for the met- 

 amorphic period. The brain ventricle opens into the mouth 

 cavity and remains in the adult as the opening of the neural 

 gland. The adult brain ganglion develops as a new structure 

 from the cells roofing the larval brain vesicle. The neural 



eye spot 

 statocyst^ 

 mouthy 

 atrium, 

 suckers 



thickens to form cerebral ganglion 

 atriopore (bilateral at this 



notochord 



.free cells in tunic 



mesenchyme 

 pharyngeal pore 

 endostyle 



Figure 1 -9. Diogrammotic summary of anatomical features of larval C/ave/ina. 



THE PROTOCHORDATES • 1 1 



