70 MICROSCOPICAL STUDIES. 



pharynx become multiplied great ly, and most significant of all, eye 

 and otolith vanish utterly, the well developed nervous system being 

 reduced to a fairly large ganglionic mass occupying the position of the 

 cerebral vesicle of the larva, i.e. midway between the mouth and the 

 atrial opening. Radiating nerves to the various organs are thence 

 given off. The course of these changes brings about a certain alter- 

 ation of placing of the two external apertures, as may be seen in the 

 diagrams ; thus the mouth in the fixed larva of Aplidium is at first 

 not terminal, but gradually becomes so, thus necessitating a corres- 

 ponding travel of the atrial opening. There is, however, no real 

 travel of these parts, simply a more rapid growth than of the opposite 

 side, of that part of the body wall and test lying below the mouth, 

 i.e. between it and the point of attachment to the rock. This 

 naturally pushes the mouth upwards, and finally places it in a 

 terminal position, at the point of the body furthest removed from 

 the point of fixation. Budding produces a cake-shaped colonial mass 

 of a general pink colour, flecked with white points indicating the 

 position of the mouths of the various individuals or ascidiozooids. 



With the assumption of adult form by Simple Ascidians, the 

 genital glands develop, but among the Compound, a frequently long- 

 continued course of budding, takes place prior thereto. All species 

 are hermaphrodite, but as a rule the male and female organs do 

 not ripen at the same time. The young produced are the tadpole 

 larvae we have above described. 



Only within the last 28 years have the Tunicates bulked with 

 large importance in scientific ken. Until 1866, when the Russian 

 naturalist Kowalewsky published his famous researches and specu- 

 lations upon the embryology of the group, they occupied a position 

 of comparative isolation, with ill-understood affinities, and were 

 generally treated as aberrant forms of little importance. Some were 

 for placing them among the Mollusca, largely upon the count of their 

 acephalous (headless) condition ; others from the peculiar looped 

 form of the alimentary canal would have that they were peculiarly 

 developed relatives of the Polyzoa. The lowly development of the 

 venous system favoured alike either of these views. 



But Kowalewsky 's researches altered all that, and gave the 

 Tunicates a status of great importance. He pointed out how the 

 adult Appendicularias and the tadpole larvaa of the fixed Ascidians 

 bear many close resemblances to the lower vertebrates, especially to 

 the Lancelet (Amphioxus), such for instance as the presence in both 

 of a stiff central axis, the notochord, and the occurrence of a central 

 nervous system expanding into a large cerebral vesicle at the anterior 

 end. Finally he pointed out the perfect concordance of the perfor- 



