16 



ZOOLOGY 



their maioins with the margins of the apei'tiires of the test, and 

 rcjiiiid the openings are the strong sphincter muscles by which 

 closure is effected. In the rest of the mantle the muscular fibres 

 are arranged in an irre'gular network, crossing one another in all 

 directions, but foi' the most part either longitudinal or transverse. 

 Within the body-wall is a cavity, the atrial or 'pcribranchial cavity 

 (atr. cat-.), communicating with the exterior through the atrial 



or.op 



goTLod. 



ffono 



oe.s ap 



martt 



r,; ntT„^ H ■*^"f'*'* f!;°'." ","= "el^' ■''*''''• The greater part of the test and 



oTh lfH,» ■ f ™'^<'^.';d from tliat side so as to bring into view the relation of these layers 

 ^ llnl^tn , ■'"''.5'°^^^'"**''°™'"'''5°* '^« alimentary eanal, etc, ,m. anus; atr af. 



nf ^t ,'i?l^ 1^^^'. f d"^'y,le ; ,»«. gonad ; .mod. gonoduct ; hup. neural gland ; ft,, ,. </. duct 

 ofn^aWnrc ,, "Sh»-r,; / ''"^ T' "'^"<=-S-™glioi 1 «■'■ "-V- aperture of .esophagus ; or. ap. 

 oial aperture , ph. pharynx ; sto„t. stomach ; te.it. tentacles ; test, test. (After Herdman ) 



aperture : this is not a ccelome, being formed to a great extent by 

 involution from the outer surface. 



Pharynx.— The oral aperture leads by a short and wide oral 

 passage (domodwum) into a chamber of large dimensions the 

 pharynx or hrancMal chamber {¥ig. IIQ , ph.) . This is a hicrhly 

 characteristic organ of the Urochorda. Its walls, which are thin 

 and delicate, are pierced by a number of slit-like apertures the 



