412 E. W. MACBRIDE, 
Balanoglossus renders this impossible, and so we have a series 
of isolated tridents. 
- Morgan has, in his last paper, made the exceedingly in- 
teresting observation that in the young adult a celomic 
space is found in the gill septum in the same 
position as it occurs in Amphioxus, viz. on the 
outer side of the skeleton ; and that, further, the single skeletal 
rod found later here is represented, at this stage, by two 
distinct rods. That the synapticula have been independently 
acquired in each case I am quite prepared to believe, especially 
as they are absent in the more primitive species of Entero- 
pneusta; but one of Professor Spengel’s arguments on this 
point seems to me to be absurd, viz. that in the one case the 
synapticula carry blood-vessels, and in the other they do not. 
A similar argument would prove that the interfilamentar 
concrescences of the gills of Mytilus had nothing to do with 
those in the gills of Anodon. 
Professor Spengel’s statement that the synapticula in 
Amphioxus are segmented off from the endostyle must be 
received with great caution. He gives no adequate proof of 
it in his paper! to which he refers, and it involves such 
alarming conclusions with regard to the formation of the gill- 
slits, that it is safe to maintain a completely sceptical attitude 
towards it till further proof is adduced. 
The blood-supply of the gills of the Enteropneusta is doubt- 
less, as Professor Spengel points out, exceedingly different from 
the branchial vessels of a Vertebrate; but its differences 
depend entirely on its more primitive and undif- 
ferentiated character. It is, in fact, nothing more than a 
portion of the enteric plexus; indeed, the whole blood-system 
of Balanoglossus is the most undifferentiated one which could 
well be imagined, and it is perfectly easy to see how the 
Vertebrate or, indeed, any other arrangement could have been 
evolved from it. 
As every one knows, the alternative theory of Vertebrate 
1 J. W. Spengel, “ Beitrig zur Kenntniss der Kiemen des Amphioxus,” 
‘ Zool. Jahrbiicher Abt. fiir Anat. und Ont.,’ 1891. 
