no 
THE VOYAGE OF H.M.S. CHALLENGEK. 
end the young tentacles are seen, and in front of them hr. si. points to the ectoderm in 
the region of the future branchial siphon. This indicates the anterior end of the 
Ascidian and the middle of the dorsal surface of the larva. Outside the ectoderm (hr. si.), 
the test (^.) with its numerous large bladder cells is seen. A single sense organ is 
present, and is still large and well developed {ot.). Around it and between it and the 
tentacles are some masses of cells (c.m.), which are probably nervous. Behind the sense 
organ (nearer to the posterior end of the larva) is a distinct opening {ap.) through 
both test and ectoderm. The character of the ectodermal cells changes greatly at this 
point. Over the sense organ they are almost squamous, but as they approach 
the opening they become gradually cubical and then low columnar in form; finally, where 
they turn inwards to line the short tube leading from the aperture (see PL XV. 
fig. 13, ap.), they become tall, narrow, columnar cells. On the other side of the opening 
the same changes are seen, the flattened ectoderm cells of the posterior end of the larva 
becoming first cubical and then columnar. Probably this invagination of the ectoderm at 
the dorsal edge of the anterior end of the Ascidian is the origin of the atrial aperture. 
Further back, along the dorsal edge of the branchial sac, is seen the base of the 
urostyle {u), which is continued out into the tail, while still further back is found the 
oesophagus [oe.) leading into the globular thick-walled stomach {st.). This larva is very 
remarkable on account of the advanced state of its organisation while still in a completely 
larval condition. The branchial sac, the endostyle, and the alimentary canal are well 
developed, while the urostyle, the pigmented sense organ, and the larval adhering papillae 
are still present. 
Colella elongata, n. sp. (PL XVI. figs. 1-7). 
The Colony is elongated and rudely conical. It is not flattened, and is probably attached 
by a stalk springing from the posterior wider end. The upper free end is narrow, but 
rounded. The surface is rather uneven, but fairly smooth. No common cloaca! apertures 
are visible. The colour is a dull yellowish -grey, darker towards the lower and lighter 
towards the upper end. 
The length is 3 cm., the greatest breadth is 1 cm., and the thickness about 
1 cm. 
The Ascidiozooids are elongated antero-posteriorly, and placed at right angles to the 
surface. They are usually about 1'5 mm. in length and 1 mm. in breadth. The 
anterior end is broad, and bears both the apertures. The posterior is narrower, and is 
continued into the long narrow vascular appendage, which, turning downwards, traverses 
the common test to the base of the colony. 
The Test is soft. Its outer layer is smooth and firmer than the deeper parts, which are 
very soft and spongy. It is not very transparent, and has a greyish colour. In sections 
