384 MANUAL OF ZOOLOGY. 



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opens into the atrium. The embryo Tunicate (fig. 201, A and 

 C) is at first generally free, and is mostly shaped like the tad- 

 pole of a frog, swimming by means of a long caudal appendage. 

 In one species (Molgula tubulosa) the larval form is destitute of 

 a tail, inactive, and amoeboid, and it almost immediately at- 

 taches itself by means of little outward processes which it de- 

 velops. Several other species of Molgula are also destitute of 

 a tail when in the embryo condition ; but the embryos of those 

 species of the genus which are fixed in the adult state appear 

 to be provided with a caudal appendage. Lastly, the larval 

 caudal appendage has been shown to exhibit a cylindrical rod- 

 like body, which has been paralleled with the chorda dor sails 

 of Vertebrates. The body in question (fig. 200, A) is a kind of 

 cellular rod, which agrees with the notochord of Vertebrates in 

 giving insertion by its sheath to muscles, and which is said to 

 hold an analogous position to the nervous system. In many 

 cases, also, the caudal appendage exhibits in addition diverging 

 rays, which have been compared with the rays of the tails of 

 young fishes. 



It is impossible here to enter minutely into the structures which compose 

 the larval Tunicate, the relations of these structures to one another and 

 to the larva as a whole, or their precise homological import. Such im- 

 portant theoretical conclusions have, however, been based upon the inter- 

 pretation of these structures, that a few words may be directed to this 

 point. 



According to the researches of Kowalewsky and Kupffer, the larval 

 Tunicate differs from the Invertebrata generally, and agrees with the Verte- 

 brata in the fact that the embryo is bicavitary, the nervous system being 

 developed in a dorsal cavity, quite separate from the cavity in which the 

 viscera generally are formed. The axial structures of the tail, as described 

 by the same observers, are stated to commence as a double row of quad- 

 rate cells, surrounded by oval muscle-cells, and projecting slightly into the 

 body of the larva, having the visceral canal below, and the neural canal 

 above. When complete, the axis of the tail is said to consist of a carti- 

 laginous elastic axial rod, surrounded by a cellular envelope. According, 

 then, to the views of the observers just mentioned, the axial rod of the 

 tail of the larval Tunicate is the precise equivalent of the "chorda dor- 

 salis " of vertebrate animals, seeing that it is dorsal in position, and that 

 it is intermediate between the neural and visceral cavities. The perforated 

 branchial sac being, further, regarded as a development of the anterior 

 portion of the alimentary canal, is an additional support to this view ; as 

 it can be compared with the dilated and perforated pharynx of the Lance- 

 let (Ampkioxus) the lowest of the Fishes. 



The views sketched out above, though accepted and endorsed by many 

 high authorities, have not been allowed to pass without question. The 

 opinions originally put forward by Kowalewsky as to the essential identity 

 of the developmental processes of the Tunicates and the Vertebrates have 

 been attacked, and many of the most important of his alleged facts have 

 been denied by such well-known authorities as Mecznikow, Donitz, Giard, 

 Von Baer, and Reichert. Without entering into this controversy further, 



