MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
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in size, with the form described as Salpa B. One-half therefore of Chamisso’s theory 
is clearly coi-rect ; the solitary Salpa (Salpa A) produces the aggregate form (Salpa B); 
and we may add, that this takes place by a process of gemmation from the walls of a 
tube in free communication with the circulatory system of the parent. 
28. Solitary Foetus of Salpa^. — Whilst this form still forms part of the chain or is 
but just freed, it is sure to contain a solitary foetus ; and frequently one may be found 
in it when it has attained its full size, but as often not. 
When the solitary foetus exists, it hangs freely in the respiratory cavity (Plate XV. 
figs. 4, 8) by means of a pedicle attached to the upper and posterior part of its wall, 
on the left side of the mouth of the parent. In its youngest and most rudimentary 
state it is a somewhat conical papilla (Plate XV. fig. 7) or bulging of the inner tunic, 
consisting of an inner oval or pyriform cellular mass, enveloped in a delicate trans- 
parent membrane, which appears to be a continuation of the inner tunic. 
As development proceeds the inner mass becomes divided into two portions, a 
larger turned towards the respiratory cavity, and which projects more and more into 
it, and a smaller subspherical, turned towards and lying in the cavity of the sinus, 
and bathed by the parental blood. 
29. The whole mass goes on enlarging, but the former portion faster than the 
latter. The former becomes somewhat ovate, with its long diameter in the same 
direction as the long diameter of the parent, and gradually assumes the form of a 
Salpa. The muscular bands, the gill, the ganglion and its otolithic sac become 
distinct, and eventually the heart is obviously seen pulsating close behind the pedicle 
of attachment, Plate XVI. fig. 6. 
In the meanwhile the smaller subspherical mass has undergone a remarkable 
change. It has likewise become thrust from the sinus towards the respiratory cavity, 
so that it no longer lies in the former, but is situated in the thick pedicle of the 
young Salpa. 
It has furthermore become hollow, and contains two perfectly distinct cavities or 
sacs ; of these the outer is concave and cup-shaped and envelopes the inner, which is 
subspherical, Plate XVI. fig. 6 m. Now the outer sac is in free communication by 
a narrower neck, divided into two channels by a partition, with the dorsal sinus of 
the foetus ; and the inner sac is in equally free communication by a neck similarly 
divided, with a short sinus arising immediately behind the heart ; and as there is no 
communication between the two sacs, it follows that the current of blood in each is 
perfectly distinct from and independent of, that in the other. A more beautiful sight 
indeed can hardly be offered to the eye of the microscopic observer than the circula- 
tion in this organ. The blood-corpuscles of the parent may be readily traced enter- 
ing the inner sac on one side of the partition, coursing round it, and finally re-entering 
the parental circulation on the other side of the partition ; while the foetal blood- 
corpuscles, of a different size from those of the parent, enter the outer sac, circulate 
round it at a different rate, and leave it to enter into the general circulation in the 
dorsal sinus. 
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