2 PHYLUM TUNIC ATA (UROCHORDA). 



The Tunicata. are entirely marine animals. They comprise 

 sessile forms; Attached to foreign bodies in the sea, and pelagic 

 f orms .wteK float 'freely, principally in the surface waters of the 

 o:-eji v . 'litany* of 1 , them live in colonies and most possess the 

 power of asexual increase by budding. 



Their place in the system was for a long time unsettled. By 

 Linnaeus they were placed partly amongst the Mollusca and partly 

 amongst the Zoophyta, and this example was followed until the 

 beginning of the last century, when as a result of the anatomical 

 investigations of Cuvier, Lamarck and Savigny, the true nature 

 of the compound Ascidians and their relationship to the simple 

 Ascidians were recognized. As a result, the group Tunicata was 

 established by Lamarck in the year 1816,* and placed between 

 his Radiata and Vermes. Lamarck recognized their distinctness 

 from the Mollusca, but Cuvier (Regne Animal, 1817) failed to do 

 this and placed them with the Acephala. The views of Cuvier 

 predominated and the Tunicata were regarded as Molluscs until 

 the middle of last century. At about that time suspicions arose 

 as to the correctness of Cuvier's view, and the class Molluscoidea 

 was in 1844 suggested by H. Milne-Edwards to comprise the 

 Polyzoa, Brachiopoda and Tunicata. At this point the discus- 

 sion remained, until the publication in 1866 of Kowalevsky's 

 work on the development of the simple Ascidians. He there 

 showed that the tailed larva, first observed by Milne-Edwards 

 and afterwards described by Krohn, is evolved by a process 

 closely resembling the early development of the vertebrate 

 embryo, that it possesses a dorsal tubular nervous system 

 and a notochord, and that the branchial basket work of the 

 adult is derived from simple stigmatic apertures which can be 

 fairly brought into relation with the gill-slits of the Verte- 

 brata. Various far-reaching phylogenetic hypotheses have been 

 based upon this discovery, amongst the most astonishing of 

 which must surely be ranked that which assigns to the Tunicata 

 a place in the direct line of vertebrate ancestry. But putting all 

 these on one side, as indeed we may safely do without any 

 appreciable loss to zoology, there still remains this important 

 result from Kowalevsky's famous investigation : it has settled 

 finally the systematic position of the Tunicata. Henceforth 

 they must be placed, if not actually within, still in close association 

 * Histoire naturelle des animaux sans veriebres, 3, 1816. 



