ON A TORNAEIA FOUND IN BRITISH SEAS. 67 



Kowalevsldi, describes the collar- and body-cavities as arising- from 

 separate pairs of arclienteric pouches. The account given above^ 

 which was first given by Metschnikoff and has since been proved by 

 Spengel, is in accordance with Batesou's observations^ if I am right 

 in considering that the hind-gut is in reality a portion of the true 

 gut and not a proctodaeum. But although I believe that the evi- 

 dence at my disposal is opposed to the view that this region of the 

 alimentai'y tract is a pi'octodseum, Prof. Spengel informs me that he 

 is inclined to think that it is^ and should he prove to be correct, the 

 origin of the mesoblast from such a source would be without parallel 

 in the animal kingdom. I expect to find that my view is correct, 

 because a widely different origin of the mesoblast in two species of 

 the same genus is in itself hardly credible, and would present most 

 serious morphological difficulties. 



The apical sense organ in our Tornaria is shown in section in 

 fig. 18. Its central portion is composed of columnar sense-cells 

 bearing cilia. Outside of these are larger cells, with large nuclei 

 surrounding a pair of deeply pigmented pits. These pits are the 

 " eye-spots " of previous authors, the large cells surrounding them 

 are probably ganglion-cells. Beneath the sensory cells is a thin 

 layer of nerve-fibres. The structure of the sense organ and its 

 relation to the muscular band which connects it with the proboscis 

 cavity can readily be seen in fig. 18. The paired invaginations 

 forming sense pits, suggest a comparison with similar organs in the 

 unarmed Gephyrea. 



Unfortunately, the account of the British Tornaria must stop 

 here. No specimens older than that described were taken during 

 the year. Possibly if we had brought back our Tornaria alive we 

 might have succeeded in rearing the further stages as was done by 

 Metscbnikoff and Agassiz, but owing to the distance we had to 

 traverse in a small sailing-boat, and that often in calm weather, we 

 found it always expedient to preserve our catch on board. Very 

 probably the Tornaria ceases to lead a pelagic life, sinks to the 

 bottom, and undergoes its further development there, which would 

 explain our taking no later stages in the tow-net. 



In conclusion, I have to thank Prof. Spengel for his kindness in 

 sending me proofs of his forthcoming illusti-ations of the develop- 

 ment of Balanoglossus. From these it appears that he has already 

 anticipated anything that is new in this paper, such as the formation 

 of the " heart " and the structure of the sense organ. I am also 

 indebted to Mr. Weldon for lending me numerous drawings and 

 preparations of the two forms of Tornaria found by him in the 

 Bahamas, and to Mr. Rupert Vallentin for the specimens taken by 

 him at Falmouth. 



