1 IS 



CHORDATA 



(fig. 503, C). At the beginning of the last century the poet Chamisso discovered 

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

 turn came from the chain form, the first instance of alternation of generations. 

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

 budding cone or stolo prolifcr from which colonies of salps bud one after another. 

 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 the solitary and the chain forms have received names, the species 

 oiSalpa* now have double names likeSalpa dcmocratica-ir.ucronata, democrat ua 



being the asexual, m-ucronala the sex- 

 ual, individual, etc. From the true 

 Snip? Doliolum* is distinguished 1 y 

 the better developed gills, the complete 

 muscular bands, and a more compli- 

 cated alternation of generations. 



SUB PHYLUM III. ENTEROPNEUSTA 

 (HEMICHORDIA) 



The few marine forms here in- 

 cluded are decidedly worm-like, and, 

 like many worms, they burrow in the 

 mud. The body consists of three 

 parts proboscis, collar, and trunk 

 (fig. 506). The proboscis, which sits 

 in the collar like an acorn in its cup, 

 whence Balanoglossus, 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 alter- 

 nately enlarging and contracting these 

 parts the animal is able to burrow. 

 The mouth (fig 505) lies ventral and 



front of the collar and leads into a 



L h 



cc 



CO 



n 



n 



FIG. 505. Sagittal section of (Jlossj- 

 balanus mimitus (diagram after Spengel). 

 c, proboscis coelom; cc, colar ccelom; co, 

 collar; h, so-called heart; /?, long muscles; 

 ', '-, dorsal and ventral nerve cords; o, 

 oesophagus; p, proboscis; nc, so-called 

 notochonl; , gill slits; v 1 , v 2 , dorsal and 

 ventral blood-vessels; m, mouth. 



in 



digestive tract, which in its anterior 

 part is perforated by numerous paired 

 gill slits, whence the name Entero- 

 pneusta, while the part behind it is 

 covered with hepatic caeca. The in- 

 testine is supported in the coelom 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 contractile 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 produced by 

 inrolling, as is the central nervous system in the Chordates, 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 regions and open- 

 ing to the exterior. 



The systematic position of the Enteropneusta is uncertain. In the possession 

 of gill slits and in the formation of the dorsal nervous system it closely resembles 

 the other chordates, and the resemblance is strengthened by similarities in details 

 of structure of the gills. The advocates of this view recognize the notochord in 



