57 ° 
SEMIVERTEBRA TES. 
the surface of the giant Pyrosoma, as it lay on deck in a tub at night, and my 
name came out in a few seconds in letters of fire.” 
Non-Luminous With the exception of the family just mentioned, and also of a 
Pelagic Ascidians. seconc [ one which constitutes the third order, the present ordinal 
group—termed the Thaliacea—includes the whole of the free-swimming pelagic 
representatives of the class. Either simple or compound in structure, these 
ascidians lack both a tail and a notochord in the adult, but have a persistent 
outer tunic, which may be either feebly or fully developed. In the inner tunic 
the muscles are arranged in the form of more or less nearly complete circular 
bands, the contraction of which forms the motive agency of the creatures. The 
branchial chamber has either two large openings, or a number of smaller gill-slits, 
leading to a single atrial cavity; the latter communicating with the exterior by 
the exhalent aperture, and the vent opening within it. In all the members of the 
group an alternation of generations takes place; and this may be further com¬ 
plicated by the individuals of a single generation being unlike one another. 
During one period of existence temporary colonies may be formed, but these never 
increase by the budding of the constituent units, which eventually separate from 
one another and disperse. 
AN INDIVIDUAL OP A CHAIN-SALPA. 
a, iulialeut, and b, exhalent, orifice ; d, gill; c, e, viscera; /, eye(?); g, pedicle of union (nat. size). 
The well-known salpte form a suborder—Hemimyaria—characterised by the 
formation of temporary colonies in the sexual generation, and represent a family 
(,Salpidce ) distinguished by the muscular bands of the inner tunic being incomplete 
on the lower surface of the body. Pelagic in habit, and transparent in structure, 
salprn have been not inaptly compared to a barrel with both ends knocked out; 
and really consist of little more than a huge pharynx, swimming through the 
water, and taking in large mouthfuls of the same at each contraction of its 
muscles. Through the hollow, to below the hinder aperture, runs obliquely a 
rod-like gill ( d ) from above the mouth, although this is too narrow to interfere 
with the free flow of the water; while the lower surface of the interior of the 
creature is furnished with a ciliated slime-secreting band, corresponding tg the 
structure known in other ascidians and the lancelet as the endostyle. It may here 
be well to mention that in the lancelet the structure in question is an elongated 
gland situated at the base of the pharynx, and against which the ends of the 
