REVIEW OF SPENGEL’S MONOGRAPH ON BALANOGLOSSUS. 418 
descent is Dohrn’s famous Annelid theory. The most striking 
and, indeed, almost the only point of similarity between 
Annelids and Vertebrates is the metameric repetition of many of 
their organs which both exhibit. Bateson, in criticising this, 
pointed out that whereas in Annelids this metamerism depends 
on the repetition of mesoblastic segments, in Vertebrates the 
repetitions of certain sets of organs have taken place indepen- 
dently of each other ; the series of gill-sacs, for instance, bears 
no relation to the series of myomeres. Spengel has made the 
daring attempt to explain away this well-known fact. He says 
that the gill-slits of Amphioxus are at first in strict correspond- 
ence with the myomeres. Considering the fact that this remark 
can only by any possibility be applied to the first dozen slits, 
and that they even arise long after the complete ventral fusion 
of the myomeres, it is difficult to see why any weight should 
be attached to a chance correspondence between the first few 
slits and the myomeres adjacent to them. Spengel’s only 
evidence for this correspondence, such as it is, consists in 
some figures of Amphioxus larve in Willey’s paper,! and if 
Professor Spengel had taken the trouble to count the slits 
and myomeres instead of resting satisfied with a superficial 
examination of the figures, he would have seen that the corre- 
spondence on which he relies does not exist.” 
Having thus seen that no valid objections to the homology 
of the nerve-chord, notochord, and gill-slits of Amphioxus with 
the similarly named structures in Balanoglossus have been 
brought forward, we are the less concerned to defend the other 
homologies put forward by Bateson. No one, we suppose, will 
deny that the “ tongue-bars” are similar structures in both 
animals, The ‘‘ digging” mouth is probably in each case an 
? Arthur Willey, ‘“‘ The Later Larval Development of Amphioxus,” ‘ Quart. 
Journ, Micr. Sci.,’ 1891. 
2 Professor Lankester has kindly looked carefully into this matter for me, 
and he writes me that at the time when three gill-slits are present in the 
larva if one counts the club-shaped gland as a fourth and makes allowance 
for the obliquity of the myotomes, an apparent correspondence exists between 
the two sets of organs. Such are the dimensions to which Spengel’s “ strict 
metamerism ” of the gill-slits reduce on examination. 
