ON THE DIPLOCHORDA. 341 



Of the two preceding workers, the keynote of McIntosh^s 

 (2) views was a close resemblance to Phoronis and Rhab- 

 dopleura; of Harmer's (1), an alliance with Balano- 

 glossus. Whilst accepting the views of the latter, I think 

 the work below will justify the placing of Phoronis and 

 Cephalodiscus together under the title Diplochorda. 



We may add that Professor A. Lang (4), in an article upon 

 the organisation of Cephalodiscus, accepts its affinities with 

 Balanoglossus. Further than this, he considers that the 

 organs of the former show an earlier stage of evolution than 

 those of the latter. On the other hand, he comes to the con- 

 clusion that the simplicity in structure of Cephalodiscus is 

 not primitive, but has been secondarily acquired by a degenera- 

 tion due to its habit. I fail to see any indications that this is 

 the case. It seems to be the more natural conclusion, and 

 that most in accordance with the facts — more especially since 

 I hope to show (below) that a distinct vascular system and 

 also paired notochords are present — to regard this animal as 

 retaining a primitive Archichordate organisation, always ex- 

 cepting the dorsal flexure of the gut, and the hypertrophy and 

 forward direction of the tentacles, which cannot be regarded as 

 characters of a free-swimming chordate ancestor. This will be 

 further referred to below, 



Lang considers the resemblance of Cephalodiscus to 

 Phoronis and the Bryozoa to be convergent. Whilst 

 leaving out of the question at present the latter, I hope to show 

 sufficient anatomical resemblance between Cephalodiscus 

 and the free-swimming Actinotrocha to outweigh such 

 suggestions. 



The absence of pharyngeal clefts apparently extends to 

 Rhabdopleura as well as Phoronis, but the value of this 

 feature cannot be gauged till we know more concerning the 

 origin of these organs. 



Nervous System. — McLitosh (2) described the main 

 nervous ganglion on the dorsal side of the collar region, and 

 remarked, " It extends for a considerable distance laterally 

 on each side along the basal region whence the plumes 



