MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
573 
diminishes, they frequently accumulate around the intestine in consequence of be- 
coming entangled among the meshes of the areolar tissue (13.) connecting the 
intestine with the parietes. 
21. So far, the structure of the two forms A and B has been identical ; but in pro- 
ceeding to examine the reproductive organs, it will be necessary to treat of each 
separately. 
The form A is always found to possess a connected series of young forms, the so- 
called Salpa chain, encircling its visceral nucleus ; the form B, on the other hand, 
never possesses the Salpa chain, but generally contains a solitary foetus, pendent from 
the upper and posterior part of its respiratory cavity. It is clear therefore that in 
each of these forms reproduction takes place. But is the mode of reproduction in 
each case similar or different? Are both, processes of gemmation, or processes of 
sexual reproduction, or is one process of the one description, the other of the other 
description ? To come at the solution of this question, it will be necessary to know 
first, the nature and relations of the chain of young in A, then the nature and rela- 
tions of the solitary foetus in B, and, finally, to trace back the development of both 
to their first origin. 
22. Salpa chain of A (Plate XV. fig. 1 h. Plate XVI. fig. 1. Plate XV. fig. 9). The 
chain is formed of a double series of fetuses, commencing on the right side of the 
nucleus, curving under it, then turning upwards and over it to the right side, and 
finally terminating in the middle line by a free extremity midway between the two 
long posterior horns. 
The chain is enclosed in a proper cavity, hollowed out in the substance of the outer 
tunic, and this sometimes opens externally opposite the free extremity of the chain, 
Plate XV. fig. 9. 
23. The fetuses do not form a chain by mere apposition ; they are all attached 
by pairs to one side of a cylindrical double-walled tube, which is connected, at its 
anterior or proximal extremity, with the system of sinuses of the parent, to the right 
of the heart. The tube is in fact merely a diverticulum of the sinus system, Plate XV. 
fig. 9, and the blood contained in the sinuses passes freely into it. It is divided by a 
partition {y) into two canals, which are distinct for the whole length of the tube, 
except at its very extremity, where they communicate just as the two scales of the 
cochlea do ; and it thence happens, that in the living animal, a constant current 
passes upon one side of the partition and down on the other, the direction of the two 
currents being generally, but not always, reversed with the reversal of the general 
circulation. 
If the fetuses be traced back upon this tube, it will be found that towards the 
proximal end of the tube they lose their distinctive form and become mere buds, 
processes of its wall, Plate XV. fig. 9. Itmay thence be conveniently termed the 
“ gemmiferous tube.” 
24. The proximal extremity of the gemmiferous tube is simply transversely striated, 
MDCCCLT. 4 E 
