2O2 



THE ASCIDIANS. 



f 







oxus, but it is merely due to the outgrowth of the tail from 

 the body of the embryo (Fig. 101). 



The structures involved in the outgrowing tail are the 

 dorsal nerve-tube, the notochord, the caudal mesoderm, 

 which lies on each side of the notochord, and will give rise 

 to the muscles of the tail, and finally a solid cord of endo- 

 derm consisting of two rows of cells placed side by side 



below the notochord (Fig. 

 99 *) As soon as the tail 

 begins to grow out, the neu- 

 renteric canal becomes ob- 

 literated, and shortly after- 

 wards the anterior neuropore 

 Fig. 101. -- Embryo of Phaiiitsia closes up temporarily. At a 



mammillata in side view, to show com- . . 



mencing outgrowth of tail. (After later period, as we shall see, 

 KOWALEVSKY.) ft re opens ; not, however, to 



ch. Notochord. ec. Ectoderm, en. En- 



doderm. mes. Mesoderm ; the cells in- the exterior, but into the 



dicated by dark outlines, beneath which , . , 



may be seen the notochord and caudal DUCCal tube. 



endoderm. n.p. Neuropore. n.t. Medul- ^ g \\\Q tail grOVVS in 

 lary tube. 



length, it becomes coiled 



round about the body of the embryo, attaining two or 

 three times the length of the latter. 



The cord of endoderm cells in the tail of the Ascidian 

 larva has been supposed to represent a rudimentary intes- 

 tine homologous with the straight intestine of Amphioxus, 

 the larval tail being on this view equivalent to the 

 post-branchial portion of the trunk in Amphioxus. This 

 view, however, is probably not correct, although there is 

 something to be said in favour of it. The probability is 

 that the tail of the Ascidian larva or tadpole, as it is often 

 called, is an organ which has been specially elaborated in 

 the course of its evolution for the particular benefit of the 

 Ascidians, since (exclusive of the pelagic forms) it is their 



