in. 6 DEVELOPMENT OF ASCIDIANS 69 



muscle-cells contain cross-striated myo-fibrils at the periphery, these 

 being continuous from cell to cell. 



The nervous system is formed by folds essentially similar to those 

 of vertebrates, making a hollow, dorsal tube, extending into the tail 

 and enlarged in front into a cerebral vesicle, within which is an 

 ocellus and also a unicellular otolith (Fig. 37 a). Nerve-fibres proceed 

 only to the front end of the rows of muscles and the rest of the cord 

 contains no nerve-cells or fibres (Fig. 38). 



The larva takes no food and the gut is not well developed. There is 

 a pharynx with usually a single pair of gill-slits opening into an 

 atrium, which develops as an ectodermal inpushing. Below or around 

 the mouth various forms of sucker are formed. 



The whole process of development occupies only one or two days, 

 and the larva, in the species in which it is set free, is positively photo- 

 tropic and negatively geotropic and so proceeds to the sea surface. 

 But its life here is also limited. Within a day or two, depending on the 

 conditions, its tropisms reverse so that it passes to the bottom, turns 

 to any dark place and thus finds a suitable surface. It attaches by the 

 suckers, loses its tail, develops a large pharynx, and grows into an adult 

 ascidian. Presumably its short life in the chordate stage is sufficient 

 to ensure distribution, and the simple nervous system serves to find 

 a place in which to live. 



In addition to the sexual reproduction, tunicates have great powers 

 of regeneration and also often multiply by budding. The bud consists 

 of an outer epicardial, mesenchymal, pharyngeal or atrial tissue. The 

 epidermis develops only more tissue like itself and all the other tissues 

 are formed from the inner mass. This occurs by a process of folding 

 to make a central cavity; the nervous system, intestine, and peri- 

 cardium are then formed by further foldings. The bud thus begins in 

 a condition comparable to a gastrula but develops directly into an 

 adult, without passing through the tadpole stages. The fact that a 

 complete new animal is thus formed from one or two layers shows 

 that the separation into three layers during development does not 

 involve any fundamental loss of potentialities, as would be required 

 if the 'germ layer' theory held rigorously. The germinal tissue of the 

 bud is not necessarily derived from that of the parent. 



6. Various forms of tunicate 



Besides some 2,000 species of sessile tunicates, about 100 species 

 have become secondarily modified for a pelagic life. These pelagic 



