GENERAL CONSIDERATIONS ON THE TUNICATES. 521 



disappearance of the segmentation of the body. Only in the caudal 

 region of the A.scidian larvae and of A/>jn'u<lini/<ir/<i are there any 

 traces of the segmentation which must have been present in their 

 ancestors. In the caudal portion of the nervous system in the 

 Ascidian larva, spinal nerves are given off segmentally, as KuPFFER 

 first noticed. In Appendicularia these are connected with paired 

 ganglionic swellings in the dorsal cord. Langerhans (No. 2), by 

 the use of reagents, was further able to prove that the caudal muscu- 

 lature breaks up into ten consecutive muscle-segments (myomeres) 

 which are provided with segmentally arranged pairs of motor nerves. 

 In individual cases Langerhans (Xo. 2) found that, in the posterior 

 caudal region of Appenddcularia, the spinal nerves of the left side 

 have shifted somewhat forward as compared with the corresponding 

 nerves of the right side. A similar condition is found in Ampkioxus. 

 In the anterior region of the body, on the contrary, no traces of 

 segmentation are retained. 



Although we must recognise a great general agreement in struc- 

 ture and development between Ampkioxus and the Tunicates, it is 

 very difficult to establish exactly in detail the homologies between 

 the organs of the two groups. A special attempt of this kind was 

 made by van Beneden and Julin, but we are not able to regard all 

 their deductions as convincing. Starting from the view that, in the 

 Tunicates. a large part of the alimentary canal (in the caudal section) 

 has degenerated, van Beneden and Julin regard the rectum and the 

 anal aperture of the Tunicates as new accpusitions which are not 

 homologous with the homonomous structures of Ampkioxus. They 

 find the homologue of the rectum of the Tunicates in the " club- 

 shaped gland " of Ampkioxus, which belongs to the first trunk-metamere 

 (p. 549) and opens out near the mouth/' - Consequently the whole of 

 the precaudal part of the body in the Ascidian larva corresponds 

 to only the small anterior region of Ampkioxus, viz., to the anterior 

 cephalic part— the first trunk -segment, This view leads these 

 authors further logically to deny the strict homology of the endostyle 

 with the hypobranchial groove of Ampkioxus. Since, however, in the 

 Cephalochorda and the Vertebrata also, the anal aperture has evidently 

 undergone a secondary shifting forward and the caudal section of the 

 alimentary canal degenerates, there is nothing which compels us to 

 doubt either the homology of the rectum throughout the Chordata, 



*[\Villf.v (No. XXXVII.) regards the club-shaped gland as the right 

 primary gill-slit. — Ed.] 



