MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
569 
lateral surfaces there is, on each side, a short process. From about midway between 
the upper and lower edges of this surface, along, conical process curves upwards and 
backwards ; these processes are distinguished from the others by containing a csecal 
process of the system of sinuses in their base {y). 
Close to the lower edge of the lateral surfaces there is another short process like 
the uppermost one. 
The respiratory apertures are wide and provided with valvular lips. The posterior 
{h) is narrower, and has the valvular lip more marked. 
The ganglion {d) is less than one-fourth of the length of the body distant from the 
anterior respiratory aperture. 
The otolithes are four in number, hemispherical, and with a dark, blackish brown 
coloured spot on their external surface, Plate XVI. fig. 5. 
The endostyle (c) is nearly half the length of the body (reaches as far as the sixth 
muscular band, counting from before backwards). 
The outer surface of the integument is everywhere covered with minute asperities, 
like little prickles. 
The muscular bands (k) are seven in number, and, with the exception of the ante- 
rior and posterior, completely encircle the body of the animal. This form was always 
free and solitary*. 
8. Salpa B, Plate XV. fig. 2, on the other hand, is thus characterized. The body 
is subovoid, smaller at the posterior extremity than at the anterior {a) ; the former 
ends in a point, the latter in a small square facet. 
The sides are flattened into several irregular facets, and the upper and lower edges 
are sometimes somewhat carinated. The apertures are similar in general structure to 
those of the form A, and the anterior and posterior extremities project considerably 
beyond them. 
The ganglion {d) is placed at about one-fourth of the length of the body from the 
anterior extremity. The otolithes resembled those of A. 
The endostyle (c) is not nearly equal in length to half the body ; it does not extend 
so far back as to the third muscular band. 
The outer surface of the integument is smooth. 
The muscular bands {k) are five in number, and none of them encircle the body of 
the animal, the dorsal extremities being always separated by a considerable interval. 
This form, when young, was sometimes found in chains ; the adults were always 
separate. 
These forms, it will be observed, are widely different, and the difference is as great 
between the youngest forms of each as between the adults, so that they are not de- 
rived from one another by any species of metamorphosis, properly so called. 
Whatever be their external differences, however, their internal organization is so 
similar that the same description applies to both. 
* The statements of Meyen {op. cit.) to the contrary are certainly erroneous. 
