12 MICROSCOPICAL STUDIES. 



The form figured was, but a short time previous, attached to the 

 parent. At pi is seen a mass of cells marking the unabsorbed remnant 

 of the placenta ; el is a collection of large cells which are believed to 

 represent the urochord of the tail of an appendicularian-like ancestor. 

 It would thus also be homologous with the cartilage-like cord of the 

 tail of the tadpole larvae of sedentary ascidians. 



The nervous system consists of a central ganglion (g) above the 

 anterior end of the branchial septum, with numerous nerves given off 

 to the different organs of the body. Upon the ganglion is a rounded 

 mass, the eye, containing dark pigment of horse-shoe shape. A little 

 way in front, is a ciliated sac situated immediately above a tongue- 

 shaped ciliated organ languet pendant in the pharynx and of 

 sensory value. 



A heart is present beneath the alimentary canal, giving off a 

 large ventral longitudinal vessel. There is a similar vessel dorsally, 

 and the two are united by lateral branches. Vessels ramify in the 

 branchial septum. The peculiar blood circulation of Tunicates is well 

 marked in Salpa after beating a certain number of times in one 

 direction, the heart remains still for a moment and then resumes 

 pulsating, but in an opposite direction, thus reversing the blood-flow 

 completely. 



Life-history. The form under notice is asexual. The bud-shaped 

 part st is the first indication of the stolon, which in the adult develops 

 by growth and internal change into a long double-chain of tiny bud- 

 like salps (Fig. 19). When a certain size is attained, the chain breaks 

 free and swims away by rhythmic, and combined, alternate admission 

 and expulsion of water. These chain salps (Fig. 20) have quite a 

 different outward form, and a different fewer number of muscular 

 hoops. Each individual in the chain is connected with four others by 

 eight bars two to each. Diagram Fig. 21 shows the method. The 

 chain salps are fully sexual ; both male and female organs are 

 developed in each individual, but the female organs mature earlier 

 than the male. A single ovum only is produced. It develops in situ 

 being fertilized by spermatozoa that enter through the mouth of 

 the parent. It receives nourishment by the interposition of a true 

 placenta between it and the parent. It possesses a circulation 

 separate from that of the latter. When development is complete it 

 breaks loose and swims away. This is the solitary asexual form 

 we figure. In turn it again gives rise to a chain of sexual salps. 

 Thus the life-cycle continually proceeds. 



