KECENT WORK ON THE PROTOCHORDA. 281 



comiuuiiication wilh the proboscis ccelom (see Fig. 1). Tluis 

 the int(>rnal opening of the proboscis canal (end vesicle or 

 Eichelpforte) is lost, while the external opening remains ; and 

 it is probably a general rule that when these regional 

 pores change their function, or lose their function 

 and become vestiges, one of the first things to 

 happen is likely to be the closure of the internal 

 or coelomic opening.^ 



In accordance with the above considerations, I regard the 

 truncal canals of Spengelia and the atrio-ccelomic funnels 

 of Amphioxus as the vestiges of a pair of functional truncal 

 pores, which were homodynamous with collar-pores and 

 proboscis-pores. It is therefore of great interest to point 

 out that in Amphioxus there are also traces of the other 

 regional pores. 



' As I have referred to this loss of the internal opening of the end vesicle 

 of the proboscis canal, I will briefly state here what I believe to be some 

 of the potentialities of this structure. 



i. The proboscis-pore is frequently well in front of the anterior neuropore. 



ii. Sometimes it is closely associated with the neuropore. 



iii. Sometimes it opens into the medullary tube of the collar behind the 

 neuropore. 



iv. Frequently the end vesicle is prolonged behind the pore, as a csecal 

 follicle lying below the medullary tube. 



V. Combining what sometimes happens into one phenomenon, we see the 

 neuropore leading into the medullary tube, andasubneural organ opening into 

 the latter. 



vi. The entire medullary tube of the collar of Enteropneusta corresponds 

 to the cerebral vesicle only of Amphioxus and of the Ascidiau larva. 

 The spinal cord is represented in Enteropneusta by the dorsal nerve which 

 lies in the skin, and is not closed in. 



vii. The subneural gland of the Ascidian larva opens by the neuropore into 

 the dorsally placed mouth, and at tiie other end into the cerebral vesicle. 



viii. The inner or cerebral opening of the subneural gland of the Ascidian 

 larva is thus seen to correspond to the proboscis-pore, which has lost all 

 relation to the ccelom. 



ix. Hence the peculiar mode of development of the tunicate subneural 

 gland is explained, and the apparent absence of a proboscis-pore in the tunicate 

 larva is accounted for. 



X. The roots of Ptyclioderidae are related to the epiphysial complex of the 

 thalamencephalou of Craniota. 



