566 WILLIAM BATESON. 
Hubrecht that the cesophageal 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. Mie. 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 Biologie,’ 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-slts 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 
Benedeu 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 im communication with the atrial 
