372 TUNICATA. 



connective tissue, blood-corpuscles and the first rudiments of the 

 future body-musculature. 



The free-swimming larva (Fig. 173, A) thus already shows the 

 typical organisation of the adult Ascidian. The further transforma- 

 tions which take place after fixation are therefore slight. They 

 consist of the degeneration of the provisional larval organs, the 

 further development of the organs of the adult (especially of the 

 branchial sac) and the development of the reproductive structures 

 (stolon, genital organs). 



It should be mentioned that the free-swimming larvae of the various 

 families of Ascidians differ in many respects. On this subject, we must refer 

 the reader to the descriptions and figures of LAHILI^E (No. 38) who has also 

 utilised the larval forms for systematic purposes. The above account applies 

 mainly to the larvae of the Phallusia and Clavelina. The larvae of the 

 Distomidae (Distaplia) are distinguished by their large size and by the early 

 development and separation from them of several small buds which again 

 divide (Fig. 230, p. 457), while in the larva of the Didemnidae, only one large 

 additional individual is at first developed, thus giving rise to the appearance 

 of double individuals (p. 459). The larvae of the Botryllidae are devoid of 

 the three adhering suckers ; these seem to be represented merely by three 

 conical processes. They are further distinguished by an equatorial ring of 

 dilated mantle-vessels surrounding the body. The Styelidae resemble them 

 closely, and such a ring of mantle-vessels also occurs in many Didemnid 

 larvae which otherwise are distinguished from the Botryllid larvae. by the 

 development of double individuals and the presence of adhering suckers. 



F. Fixation and Retrogressive Metamorphosis. 



The transformations which take place after fixation may to some 

 extent be considered retrogressive, as the sensory organs, the nervous 

 system, and the locomotory organs undergo degeneration. The other 

 systems of organs, on the contrary, often become more perfect. 



The free-swimming condition does not last for more than a few 

 hours. The larval tail begins to degenerate when attachment takes 

 place. According to KUPFFER, the larva attaches itself by means of 

 only one of the three adhering papillae (Fig. 173 E, kp), the other 

 two degenerating. Even the papilla that is utilised for fixation soon 

 disappears, so that the young Ascidian then seems to be attached 

 by the surface of the cellulose mantle. In other cases (Clavelina), 

 stolon-like outgrowths from the lower end of the body bring about 

 fixation. 



The degeneration of the caudal region is introduced by the detach- 

 ment of the soft parts of this region from the gelatinous mantle 

 which envelops them and their withdrawal towards the trunk-region 



