238 



\-ki;tI':ij]:ate animals. 



sciitini;^ the foie :ui(I hind limbs. Tlie mouth i.s a longitudinal 

 tissure, jilaoed at the frniit of the head, auil completely dewtitute of 

 jaws, but surrounded by a Dumber of cartilaginous tilameuts. The 

 thi-oat is ])rovided witli seA'eral leafdike tilaincuts which are richly 

 supiilied with blood, and are believed to discharge in jiart the function 

 of gills. The mouth (tig. 171, «) oj>ens ijito a dilated chamber, which 

 i 1 Considered to rej)resent the pharynx, and is ternieil the pharyngeal 

 or " branchial " sac. The walls of this chamber {h) are strengthened 

 by numerous cartilaginous filaments, between which is a series of 

 transverse slits or clcft.s, and the whole is covered with a richly cili- 

 ated nuicous niendjrane. The function of this sac is clearly re.spira- 

 t(]ry, the water from witliont being admitted through the mouth, 

 jjassing through the branchial clefts into the abdominal cavity, and 

 finally escaping by means of an ajierture ])laceil on the ventral sur- 

 face a little in front of the anus. From the hinder end of the 

 branchial .sac jirocecds the alimentary canal, which has ap]iended to 



Fi- 171.— Tlif T.:infTl,.t (A ai/.tnin us l,i„rodntus). piiliir-wl to tnirc its natmnl size, 

 n Miiiitli ; h Bi;mf-lii,il s;ir ; r/ Stniiiacli : h Divoiticiilinii rciufspiitiiii,' tin' livur ; 

 i Ijiti'stiiif : a Anns; n Notucliool ; / Radiuients nt liii-rays ; p Alulnniiii.Tl pure. 



it a sac - like organ believed to represent the liver, anil wdiich 

 terminates behind in a distinct an.nl ajiertiire. There is no heart, 

 and the circulation is entirely ctlected liy means of .several con- 

 tractile dilat.dions, developed upon the great blood-ve.ssels. The 

 blood itself is colourless. No l<idne^■s have hitherto been dis- 

 covereil, and the reprodui'ti^'C elements are emitted into the abdom- 

 inal cavity, from which they esca|)e by the |iore jilaecd upon the 

 lower surface. 



There is no skeleb>n ]iro|)erly so called. The ni>tochoi'd (n) 

 remains throughout life as a send - gelatinous rod, enclosed in a 

 i]iembr,'iii(Mis sheath, and supporting the s]iinal I'ord. There is no 

 skull, , and tin' spinal cor<l does not expand in front to form a dis- 

 tinct brain or cWfvyi//(r/o». The br.-iin, however, mav be said to be 

 ri'presented, as the front |iortion of the iier\-ons axis givi's otl' nerves 



to a pair of e\'es, and : tlier br.-ineli to a ciliated ]iil, which is be- 



lie\'ed lo be a riidimeiil a.r\' or"an of smell. 



