57' 



SEMI VERTEBRA 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. secon{ | 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 OF A CHAIN-SALPA. 



", inhalent, ami b, exhalent, oritice ; d, gill ; c, e, viscera ; /, eye(?) ; g, pedicle of union (nat. size). 



The well-known salpse 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, 

 salpae 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) Prom above the mouth, although this is too narrow to interfere 

 with the five flow of the water: while the lower surface of the interior of the 

 creature is furnished with a ciliated slime-secreting band, corresponding to the 

 structure known in other ascidians and the lancelet as the endostyle. It may here 

 he 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 



