5V2 



CHORD ATA. 



salps were f)rochiced by the solitary iiulividuals, and that these in 

 turn came from the chain form, a i^eculiar ty2)e of reproduction 

 to which iSteenstrup later gave the name alternation of generations. 

 The solitary salp is asexual; gonads are lacking, but near the 

 hinder end is a budding cone or stolo j^rolifer from which one 

 after another bud colonies of salps. When 

 the first is separated a second matures and 

 a third begins. These colonial forms, the 

 chain salps, are sexual, and each produces a 

 single egg from which a solitary individual is 

 formed. 



Since both tlie solitary and the chain forms have 

 received names, the species of Salpa* now have 

 double names like Salpa democratica-mucroiiata. 

 democratica being the asexual, mueronata the sex- 

 ual, individual, etc. From the true 8alp(T Dolio- 



FiG 55l.-Do(/ohim cienti- jyin* is distinguished bv the better developed gills, 

 cuininm. (Forexplana- , ', i , 



tion of letters see flg. the complete muscular bands, and a more compli- 



550.) 



cated alternation of generations. 



Sub PHYLUit III. Enteropsteusta (H?;michordia). 



The few marine forms here included are decidedly worm-like, 

 and, like many worms, they burrow in the mud. The body con- 

 sists of three parts — proboscis, collar, and body -(fig. 552). The 

 proboscis contains a cavity opening to the exterior by a dorsal 

 pore, while two similar cavities in the collar open separately. 

 These can be filled with water, and by alternately enlarging and 

 contracting these parts the animal is able to burrow like a razor 

 clam {Ensis). The mouth lies ventral and in front of the collar 

 and leads into a digestive tract, which in its anterior part is per- 

 forated by numerous paired gill slits, while the part behind it is 

 covered with hepatic Cfeca. The intestine is stipj^orted in the 

 ccelom by dorsal and ventral mesenteries, and is accomjianied by a 

 dorsal and a ventral blood-vessel, to which are added lateral canals 

 and numerous anastomoses. A vesicle on the dorsal vessel in the 

 proboscis is called the heart. The nervous system is very peculiar. 

 There is a dorsal portion lying in the collar region, which is pro- 

 duced by inrolling, as is the central nervous system in the Chor- 

 dates, and a ventral part, as yet lying in the ectoderm, the two 

 being connected l)y nerves in the collar. The gonads are numerous 

 follicles lying between gill and liver region and opening to the 

 exterior. 



