512 CHORD AT A. 



salps were produced by the solitary individuals, and that these in 

 turn came from the chain form, a peculiar type of reproduction 

 to which Steenstrup 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 prolifer from which one 

 after another bud colonies of salps. When 

 the first is separated a second matures and 

 en 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 the solitary and the chain forms have 

 received names, the species of Salpa* now have 

 double names like Salpa democratica-mucronata, 

 democratica being the asexual, mucronata the sex- 

 ual, individual, etc. From the true Salpce Dolio- 

 orex lana" ^ um * is distinguished by the better developed gills, 

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

 cated alternation of generations. 



SUB PHYLUM III. ENTEROPNEUSTA (HEMICHORDIA). 



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 caeca. The intestine is supported in the 

 ccelom by dorsal and ventral mesenteries, and is accompanied 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 by nerves in the collar. The gonads are numerous 

 follicles lying between gill and liver region and opening to the 

 exterior. 



