308 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



/9. Historical. In 1881 Julin published, in the Archives de Biologie (T. ii.), his 

 well-known anatomical work on the subneiiral gland of Ascidians, in which he developed 

 the idea that the subneural gland with its duct which opens dorsally into the branchial 

 sac at the base of the buccal siphon by the dorsal tubercle, is homologous with the 

 hypophysis cerebri of craniate Vertebrates'. This work was followed in 1884 by a 

 memoir published in the fifth volume of the same Archives under the joint names 

 of Ed. van Beneden and C. Julin, in which the authors sought to substantiate their 

 hypothesis by the facts of development. 



They described the origin of the subneural gland in a species of Glavelina, from 

 a simple evagination of the wall of the branchial sac, which they called the " caecum 

 hypophysaire." This method of development is of course, in the main, like that followed 

 by the hypophysis of the Craniota and would, if true, no doubt tend to support their 

 hypothesis. It would at the same time rob the Ascidian subneural gland of any 

 morphological interest that might be expected to appertain to it, because it would 

 prove identity where we might reasonably hope to find evidence of change. 



In 1892 (Zool. Anz. xv. 1892, p. 332) I showed that in Giona intestinalis and 

 Glavelina lepadiforwds the adult ganglion and the subneural gland arise from a common 

 primordium which I called the neuro-hypophysial canal. This canal opens primarily 

 at its posterior end into the cerebral vesicle, while at its anterior end it secondarily 

 acquires an opening (which may be defined as the rudiment of the dorsal tubercle) 

 into the base of the dorsal mouth. There is good reason . to interpret this secondary 

 communication with the mouth as a re-opening of the neuropore. A similar mode of 

 development has since been described in other forms by Hjort^, Salensky and Metcalf. 



Meanwhile in 1886, Bateson^ compared the proboscis-pore of Balanoglossus with 

 the praeoral pit of the larva of Amphioxus and, on the strength of Julin's anatomical 

 work, with the dorsal tubercle of the Ascidian subneural gland. Bateson's other sug- 

 gestions on this point were of course made before the development of the subneural 

 gland was known and need not be referred to here. In concluding his remarks on 

 this subject he says (p. 564), " If these views are correct the pituitary body and its 

 pore is to be regarded as the rudiment of a primitive excretory organ which originally 

 opened dorsally." As will be seen, this conclusion is borne out by facts (see below, 

 p. 314). 



Strange as it may appear, it is nevertheless true that it has generally been found 

 easier to compare the Urochorda with the Craniota than with the Enteropneusta, i.e. easier 

 to compare them with higher than with more primitive forms. 



The idea of the neuro-hypophysial canal does not appear to have gained many 

 adherents, and it is no doubt very right that it should have been so until further 

 information was forthcoming. This information is now to hand and it may be summed 

 up by saying that the pore-canal or end-sac of the proboscis of Enteropneusta is homo- 

 logous with the primordiuvi of the subneural gland of Ascidians, this primordium being 



1 The same suggestion was made in the same year by Balfour in his Comparative Embryology. 



2 Hjort dealt with Distaplia magnilarva, and his preliminary account appeared at the same time as my 

 own (Zool. Anz. xv. 1892, p. 328). 



^ W. Bateson, "The Ancestry of the Chordata," Q. J. M. S., Vol. xxvi. , 1886. See also Bateson's previous 

 papers in the same and two preceding volumes. 



