660 



CHORDATA 



CLASS 2. THALIACEA. 



The salpas (Fig. 1,004). Pelagic, transparent tunicates, more or 

 less cylindrical in shape, with a large oral opening (1) at one end of 

 the body and a cloacal opening (12) at the other. The body wall con- 

 sists of a cellulose tunic (4) and a mantle, the latter containing a 

 number of conspicuous, more or less ring-shaped muscle bands (11), by 

 the contraction of which water is expelled from the cloacal opening as 

 the animal moves forwards. The cavity of the body is divided by a 



respiratory parti- 



_y* ;( a* tion (3) containing 



J|p!3^*<^ slits or stigmata 



(Doliolum) or con- 

 sisting simply of a 

 ciliated bar (Salpa), 

 into two chambers, 

 the anterior or 

 pharyngeal (5) and 



a 



Fig. 1,004 Diagram of a salpa (Cambridge Natural 

 History). 1, mouth; 2, brain; 3, respiratory partition; 

 4, test; 5, pharynx: 6, endostyle; 7, heart; 8, nucleus; 



9, cloacal cavity ; 10, ovary ; 11, muscle bands ; 12, cloa- 4-u n r^efo* n-n 1 

 cal opening; 13, peripharyngeal ridge ; 14, dorsal lamina ; tne posterior or Clo- 

 15, salpa chain. acal ( 9 ), t h r o u g h 



which the water streams. Endostyle (6) and dorsal lamina (14) are 

 present. The principal viscera lie embedded in the mantle on the ventral 

 side of the body (8), forming there in many species a small, compact, 

 usually highly colored mass called the nucleus. The principal ganglion 

 (2) lies in the dorsal wall of the pharyngeal chamber. Immediately 

 above it in Salpa is a simple eye. Beneath the ganglion is a subneural 

 gland. 



The Thaliacea have what is usually called an alternation of genera- 

 tions, a solitary generation alternating with an aggregated one, although 

 in Salpa at least it is not strictly so. In the solitary form of Salpa a 

 ventral, posterior stolon arises, on which buds appear which are destined 

 to become the aggregated generation. Near the base of the stolon the 

 solitary individual bears a mass of germ cells, some of which are received 

 by the buds as they arise. Thus the buds, and the animals of the aggre- 

 gated generation they grow into, serve merely as nurses for the embryos 

 arising from these germs which have been received from the solitary 

 individual. The aggregated individuals form the Salpa chain and often 

 remain attached together long after they have left the parent. The class 

 contains about 40 species, grouped in 2 orders. 



Key to the orders of Thaliacea: 



di Respiratory partition plate-like 1. MULTISTIGMATEA 



o a Respiratory partition rod-like 2. ASTIGMATEA 



