44S 



CHORDATA 



(ha;. ,03, C). Al the l>c,i;inninL; of the last century the poet Chamisso discovered 

 that the chain salps were produced bv tlie solitar\- indi\iduals, and that these in 

 turn came from ilie chain f.n-m, the hrst instance of allernation of generations. 

 Tlie solitary salp is asexual; j,'onads are lacking, luit near the hinder end is a 

 budding cone or slolo proliicr from which colonies of salps bud one after another. 

 \\'hen the first is separated a second matures and a third begins. These colonial 

 forms, the chain salps, are se.xual, aird each produces a single egg from which a 

 solitary indiyidual is formed. 



Since both the snlnary and the chain forms haye recei\'ed names, the species 



of 5d.'/>j* now ha\"C double names li! 



Fig. 505. — Sagittal section of CIosso- 

 haldints wiiiutus ^diagram after Sj.iongel). 

 c, proboscis cadoui; cc, colar CLtlom; co, 

 collar; Ji, so-called heart; Ini, long muscles; 

 »\ ?r, dorsal and ycntral ner\'e cords: e, 

 cesophagus; p, prol)oscis; ;/c, so-callctl 

 notochord; i^, gill slits; v' , v-, dorsal and 

 ventral blood-vessels; in, nioulh. 



,c Salpa dcniocraliia-iiutiroiiula, ilcinocra/ica 

 being the asexual, iinicroiiala the sex- 

 ual, indi\idual, etc. From the true 

 SdlpiC I'>o!ioluiir' is distinguished by 

 the better de\elopc(.l gilis, the com]dete 

 mitscular bands, and a more conipli- 



;'' caied alternation of generations. 



Sub rn\xrM III. Exthropnevst.^ 

 (,IIi;miciiordia) 



'■' The few marine forms here in- 



cluded are decidedly worm-like, and, 

 like many worms, they luirrow in the 

 mud. The body consists of three 



" ])arts — proboscis, collar, and trtnik 

 (iig. 500). ddie proboscis, which sits 

 in the collar like an acorn in its cup, 

 whence Boliiiio;,;li'ssnf, contains a 



,1 ' ca\"ity opening to the exterior by a 

 dorsal pore, \\hile two similar ca\iiies 

 in the collar open separately. These 

 can be hllcd with water, and bv alter- 



, natcly enlarging and contracting these 

 , parts the anitnal is able to burrow. 

 The mouth (hg 505) lies ventral and 

 in front of the collar and leads into a 

 digestive tract, which in its anterior 



part is ]ierforated by numerous paired 

 gill slits, whence the name I'hitero- 

 pncusta, while the ]iart behind it is 

 co\ered with hepatic ca-ca. The in- 

 testine is supported in the cadom by 

 dorsal and \'entral mesenteries, and is 

 accompanied liy a dorsal au'l a ventral blood-x'csscl, to which are added 

 lateral canals and numerous anastomoses. .\ contractile vesicle on the dorsal 

 vessel in the proboscis is called the heart. The nervous system is very pieculiar. 

 There is a dorsal portion Iving in the collar region, which is produced by 

 inrolling, as is the cenlr;d ner\-ous system in the (."hordalcs. and a ventral part, 

 a^ yet lying in the ectoderm, the two being connected bv ner\es 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 bhitcropneusla is uncertain. In the [wsscssion 

 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 \iew recognize the notochord in 



