LEPTOCAHDIANS. 



1213 



(ligitifonu (iii'occss-likc) thickenins's (tiir. 3(i."), v(>rtic;illy 

 a.l)uve /); miuI a cuiistiuit ('UiTeiit of wattT is thus kept 

 iq) into the bi'anchial sac (respiratory cavity, tig. 364, d). 

 Tlie mouth cavity is delimited from this sac by a 

 circlet of ciri'i (tig. o()8), similar to those of tiie oral 

 margin. 



Tiie walls of the respiratory cavity, wiiicli also 

 serves, iiowcvcr, as a pharynx or rather perliaps an 

 ■ i>so])iiagus, are laterally constructed of a net\v(jrk of 

 irtilagiiious i-ods (tig. 3(ii)), closely resembling tlie 

 liomologous structure in the Ascidians; Ijut above and 

 below the respiratorj' cavity has a continuous groove, 

 tile ujiper known as the epilii-anchial, the lower as the 

 hj'pobranchial groove. The oblii(ue]y vertical rods set 

 transversely in the body are rib-like growths on both 

 sides, connected above with the outer sheath of the noto- 

 chord (the so-called skeletogenous layer), the most pri- 

 mitive rudiment of an axial skeleton. Relow (towai'ds 

 the ventral side of the animal) these rods are free, some 

 (the so-called itrimary bars) bifurcated, others (the so- 

 called secondary bars or tongue-bars) single. The con- 

 necting rods between them stand in the longitudinal 

 direction of the b(xly. Thus a. network is formed, and 

 this is faced everywhere, as the two grooves are also 

 lined, with an epithelium of ciliated cells, exactly si- 

 milar to those we have just seen in the mouth cavity. 

 But the meshes of the net are minute, hardly visible 

 even under a high magnifying power and under ordi- 

 nary circumstances do not transmit anything but the 

 clearest, purest water. In the adult state of the aiiinial 

 this water, which has now served its respiratory pur- 

 pose, passes out on the sides of the respiratory cavity 

 and into a special chamber, a secondary compartment 

 of the abdominal cavity. This chamber, which has been 

 named by K.vy Laxkester" the atrium, surrounding 

 the respiratory cavity on the sides and underneath and 

 extending some way behind it, has a special aperture 

 (the abdominal or atrial pore) Ijehind on the ventral 

 side (tig. 364, o) for the discharge of the water. Every- 

 thing else that has accompanied the water into the re- 

 spiratory cavity, and especially the food — Infusoria 

 and other minute creatures — finds its way through tlie 

 (i-sophagus j)roi)er, a tubiform prolongation of the epi- 

 l)ranchial groove, into a kind of stomach (tig. 364, just 

 in fr(jnt of /) with a long ciecal appendage (/(), and all 

 the useless matter now passes out through the straight 



intestine (/) and tlu^ anal aperture (e). 'I'he ca'cal a]i- 

 pendage projects forward on (he right side into the 

 atrium, pressed close; to the respiratory cavity. Tlie 

 whole; digestive canal, the stomach and intestine, is lined 

 with an epithelium of ciliated cells and exhibits a vibi-a- 

 tile motion, just as the intestinal canal of the woi'ins. 

 The l)lood of Brunchiosfoma is colourless; and the 

 circulation, of which little is yet known with certainty, 

 may indeed be ret'errcid to the })iscine type, for the vascular 

 system is di\ided into certain i-egions which apparently 

 lend themselves to a com})arison witli the heart and the 

 great bloodvessels of fishes; but here too we are re- 

 minded (jf the worms. Under the respiratory cavity 

 runs a long contractile canal with a contractile vesicle 

 to each ascendinff rod in the branchial network. This 



.-rftl 



Fig. 3(59. A portion of tlu' right .siile of the liranoliial bosket in 



Branchiostoiiia lunceolulnm, seen from without and |:io\verfiilly inairni- 



liril. .After VoGT and Yung. 



<j, secondary roils, so-calU'd tongue-bars; /*, primary rods. 



canal has been cejmpared to the venous heart of fishes. 

 Another iMjiitractile tube, anteriorly double (one on the 

 right side, one on the left), posteriorly single, runs along 

 the dorsal margin of the abdominal cavity, under the 

 notochord. This canal has been compared to the aorta 

 of fishes. Another similar canal, according to J. ^li'i,- 

 LEK, connects on each side these two canals in front, 

 and this has been compared to the so-called ductus ar- 

 teriosus Bofalli of tiie lower vertebrates and of the 

 higher vertebrates in their embryonic stages. .Along the 

 intestinal canal run two ducts of a venous natiu-e. The 



Quart. .Journ. .Micr. So., n. ser.. No. LIX (.Inly, 1875), p. •207; \o. CXVI (Apr.. 1889). p. 365. 



