416 INVERTEBRATA CHAP. 



must be interpreted as a reminiscence of pouch-formation, which had 

 been suppressed because the number of embryonic cells present at 

 that stage had been diminished. 



This consideration brings us to the final point. Determinate 

 cleavage, in which each cell division separates specific organ-forming 

 substances, is distinguished from indeterminate cleavage, in which 

 the individual cell is a subdivision of no importance, by the fact 

 that in the former type there are fewer and larger blastorneres 

 present at any given stage than in the latter. 



Now the Brachiopod has almost certainly got indeterminate 

 cleavage of the egg. It would be natural, therefore, to expect that 

 Brachiopoda should retain the pouch-method of forming the coelom. 

 That the coelom, according to Kowalevsky, is cut off as a pair of 

 lateral pouches in Argiope, and as a single -bilobed pouch in 

 Terebratulina, does not seem to be of any particular importance, for 

 quite similar variations in the method of coelom formation are met 

 with amongst Echmodermata. 



If we imagine that at one time the Ctenophore-like ancestor of 

 Annelida possessed indeterminate cleavage, and that the change to 

 determinate cleavage is a later specialization, then we might imagine 

 that the Brachiopoda were an offshoot from the Proto-annelidan stem 

 before determinate cleavage had been acquired. 



If this conclusion were justified, the Brachiopoda could not be 

 included amongst the Podaxonia, but would represent an analogous 

 offshoot occurring at a much earlier date ; and thus Professor 

 Morse's characterization of the Brachiopoda as "ancient cephalized 

 worms," contrasted with Gephyrea, Serpulids, etc., as "modern 

 cephalized worms," would be found to contain a large measure of 

 truth. 



Great light on many obscure questions of Brachiopodan embryology 

 would be obtained by a successful elucidation of the development of 

 Lingula, a genus which has persisted almost unchanged since early 

 Cambrian times. 



The late Professor Brooks (1878), of Johns Hopkins University, 

 Baltimore, found a free-swimming larva of this form and described 

 its metamorphosis. This larva, however, represents a much later 

 stage in development than the free -swimming larvae of other 

 Brachiopoda, so far studied, because the man tie -lobes are already 

 retroverted, and the valves of the shell and the first rudiments of the 

 lophophore are formed. In fact it bears much the same relationship 

 to the larva of Terebratulina as the Molluscan veliger larva does to 

 the Molluscan trochophore. It is to be hoped that some one of the 

 many brilliant pupils of Professor Brooks may take up their master's 

 work and carry it to a successful conclusion. 



LITERATURE REFERRED TO 



Brooks, W. K. The Development of Lingula. Chesapeake Zool. Laboratory, 

 Scientific Results of Session 1878. Baltimore, 1878. 



