226 THE VOYAGE OF H.M.S. CHALLENGER. 



The Tentacles are long and stout, they are situated on a very strong muscular ring, 

 and are about sixty in number; they are all of much the same size, a few are slighter 

 than the rest, but are not placed regularly. 



Tlie Dorsal Tubercle is cordate and very large (measuring 3 "5 millimetres antero- 

 posteriorly) ; the horns are very long, and are both coiled inwards spirally. 



Tlte Alimentary Canal is wide. The oesophageal opening is situated at about three- 

 quarters of the distance from the anterior to the posterior end of the branchial sac. 



This large species has a characteristically solid and cartilaginous test, and may be con- 

 sidered the type of the genus Pachychlcena. In both the specimens part of the ventral edge, 

 the posterior end, and a portion of each side is wanting. Still enough remains, taking both 

 sj)ecimens together, to enable us to form a pretty complete idea of the original shape and 

 proportions (PL XXVIII. fig. 6, which is formed by combining parts of the two). 



Some parts of the surface are incrusted with a brownish Alga, while Polyzoa, Hydroids, 

 Cirripedes, Compound Ascidians, &c, adhere to or are imbedded in the substance of the 

 test. The proportions of the two specimens vary slightly ; they are as follows : — 



The test, which is probably the thickest and most solid known among Simple Ascidians, 

 is light-coloured throughout ; it contains numerous blood-vessels, and, in the outer layers 

 small bladder cells and yellow pigment cells. The crystals found on the vessels in the 

 test are rod-like or more irregular, large and of a distinct yellow colour on the larger 

 branches, while on the terminal twigs they are smaller, tapering, disposed in radiating 

 bunches, and generally colourless (PI. XXIX. fig. 10). I think it is probable that all 

 these deposits are due to post-mortem changes, caused possibly by the alcohol, and that 

 they do not exist in the living test. 



The body when removed from the test is so shrunken as only to occupy about 

 half its cavity. 1 The siphons are very long, especially the branchial, and diverge at 

 an angle of more than 90°. The right (branchial) side of the body is flat, while the left is 

 enormously developed, the viscera forming a large projection. In consequence of this the 

 body appears to have three sides, — a right and two left (one dorsal and the other ventral), 

 — or in other words, is roughly triangular in transverse section. The cloaca is filled 

 with light grey, gelatinous fsecal matter. 



The branchial sac is peculiarly thick and opaque, and the wide network of larger vessels 

 on the external surface (PL XXVIII. fig. 8, e.l. and tr.) gives it a very coarse appearance. 



1 This is after about six years' immersion in alcohol. 



