98 WILLIAM BATBSON. 



Hubrecht that the oesophageal pouches of Nemertine were the 

 homologies of gill-slits, and supposing the proboscis invagi- 

 nated and around its base a quantity of nerve-tissue deposited 

 as in Balanoglossus, the proboscis would then have the same 

 relation to the nerve-ring as that found in Nemertines. Hu- 

 brecht's view of the pituitary body falls if the alternative here 

 given is accepted. Though the points of anatomical resem- 

 blance are not striking, yet when taken with the ciliated skin, 

 the ventral mouth and position of the generative organs they 

 form a basis for comparison. 



If these resemblances were found to be real the nervous 

 system of the Nemertines would have to be supposed to have 

 arisen within the limits of the group. As both animals 

 possess a nerve-plexus in the skin this does not seem impos- 

 sible. Also the excretory system lately described by Oude- 

 mans (' Quart. Jour, Mic. Sci.,^ 1885), would have thus arisen 

 as a specialization of parts of the body cavity ; since in 

 Balanoglossus this function appears to be generally distributed 

 over the body cavity, this also might be conceived. 



Of the Tunicata. — Next, since all the Chordata at some 

 period of their development agree with the larva in Stage H, 

 in possessing a dorsal nerve-cord more or less invaginated, one 

 or more pairs of gill-slits and a notochord, let us pass on to 

 Stage H, in which the notochord is forming at the anterior end 

 of the gut. From such an animal as this the Ascidians may 

 have been descended. For, as has been suggested by van 

 Beneden and Julin (' Archives de Biologic,' 1885) it may be, 

 that all the Ascidians have but a single pair of gill-slits ; for 

 that Appendicularia has only one pair is known ; while in some 

 genera the atrial cavity arises as an increase in the size of the 

 pair of ciliated chambers by which the gill-slits open ; and 

 this increase may take place in the hypoblastic half of the 

 chambers, or in the epiblastic; by the fusion of these two 

 chambers the atrial chamber of these genera is formed. Van 

 Beneden and Julin then suggest that the atrial pore is the 

 actual opening of the two fused gill-slits, and that the rows of 

 slits placing the pharynx in communication with the atrial 



