MOLLUSCA: TUNICATA. 



385 



it will be sufficient to enumerate the more important points which the 

 researches of the observers just mentioned would seem to render more 

 or less probable : 



i. The axial rod of the larval Ascidian is ventral in position, and can- 

 not, therefore, be homologically compared with the "chorda dorsalis " of 

 the Vertebrate embryo. (Von Baer.) 





Fig. 201. Development of Tunicata, A, Larva of Botryllus violaceus, greatly mag- 

 nified : a Processes for attachment ; b Mass of primitive cells from which the diges- 

 tive organs are developed ; c Circlet of eight cellular outgrowths ; d Eye-spot ; e 

 Entrance to the branchial sac \f The external structureless " test ; " g Large nucle- 

 ated cells forming the sheath of the central axis (eight rows of these cells are pres- 

 ent). B, A portion of the tail, highly magnified ; k Central axis (/"and g, as be- 

 fore). C, Another larva of the same, viewed from the side, and highly magnified, 

 showing the superior and inferior fin-like prolongations (p p) of the " test," with ray- 

 like striae (the other letters as before). D, Diagrammatic cross-section of the tail, 

 showing the position of the fins (//), and the relations to one another of the central 

 axis (h\ the intermediate cellular sheath (g), and the external structureless test (/). 

 (After Reichert.) 



2. The embryo of the Tunicates is not really bicavitary, and the nervous 

 system is not developed in a chamber separate from and lying above the 

 visceral cavity. (Donitz, Reichert.) 



3. The nerve-ganglion of the Tunicates is placed upon the ventral sur- 

 face of the larva, and does not, therefore, correspond with the cerebro- 

 spinal nervous system of the Vertebrates. (Von Baer.) 



4. The tail of the larval Ascidian is a purely provisional organ. 



5. The axial structures of the tail (fig. 201, A, B, and D) consist of a 



2 B 



