484 TUNICATES 



to warrant the conclusion that the groups most abundant in 

 kinds are so because of their greater fitness to survive or their 

 relative adaptability to external conditions". The conclusion 

 to be drawn is that the mysterious quality of varying so as to 

 give rise to new species is strong in some types and weak 

 in others, but this is quite apart from individual surviving 

 power. 



The Tunicates offer a fine illustration of the use of embryo- 

 logy in classification. Now that we know the development, 

 we can detect certain Chordate affinities in the adult Ascidian, 

 but it is unlikely that these would have been detected if ana- 

 tomists had remained ignorant of the development. This 

 remark does not apply to the few types of Larvacea which 

 retain their Chordate characters throughout life. The reasons 

 for ranking Tunicates with Chordata are to be found in the 

 larvae, and in the Larvacea. The five important points are : — 



(i) There is a dorsal nerve-cord with anterior expansion 

 (or brain); (2) from this expansion a " brain-eye" arises; (3) 

 there is an endodermic "notochord " in the region of the tail; 

 (4) there are gill-clefts opening from the pharynx to the ex- 

 terior ; and (5) there is a ventral heart. 



The progress of research since the days of Kowalevsky has 

 not weakened the confidence of zoologists in the Chordate 

 nature of Tunicates, but some of the homologies which were 

 suggested have not been confirmed. Thus, it is very doubtful 

 (Seeliger, Metcalf) whether the downgrowth from the brain of 

 the Tunicate is homologous with the Vertebrate hypophysis, 

 and very doubtful (Seeliger) whether the ventral groove or 

 endostyle in the Tunicate pharynx is comparable to the thyroid 

 gland of Vertebrates. 



The absence of segments in the larval Tunicate is surprising 

 in a type which has so many indubitable Chordate affinities. 

 There seemed for a time that there were definite traces of 

 metamerism in the tail of Oikopleura (one of the Larvacea), 

 but this has not been borne out by subsequent work. 



The tests of Tunicates afford attachment to numerous 

 animals, such as hydroids, tubicolous polychaets, acorn-shells, 

 polyzoa, and the bivalves Anomia ephippium and Modiolaria 

 marmorata. Dozens of the latter are often found deeply em- 

 bedded in the tests of Ascidia mentula. 



