■100 



COMPARATIVE ANATOMY. 



§310. 



Tlic rudiment of the branchial chamber or branchial-gut gives 

 rise in the Acopa to very highly elaborated differentiations which 

 are in harmony with the condition presented by the Copelata. In 

 the latter, two outgrowing sacs are formed which are not placed 

 in communication with the exterior, until each is met by an ingrowth 

 of the ectoderm; and so, too, in the Ascidians, two lateral sacs arise 

 by a pinching off of the branchial gut. They communicate for a 

 time with this their point of origin, but subsequently separate from 

 it, and grow up around the walls of this part of the gut dorsally, 

 until they meet and unite with one another. As a result we have a 

 cavity formed around the branchial chamber by the lumen of these 

 united sacs, the peribranchial space (perithoracic chamber of authors). 

 An insinkiug of the surface of the body approaches the point of 

 union of the two halves of the peribranchial space, and forms, when 

 it has finally broken through, a communication to the exterior, the 

 excurrent orifice. On the ventral face the separation of the two 

 spaces persists. During the concrescence of the two sacs which 

 grow up round the branchial chamber, and of the superficial in- 

 sinking, the anal aperture is gradually brought into the area of this 

 space. This region then constitutes the cloaca (Fig. 210, cl). In the 



walls of the branchial chamber there now 

 arise clefts leading into the peribranchial 

 space ; in fact, branchial slits, which con- 

 sequently have quite a different importance 

 from that of the two primary spiracula. 



Gradually, the entire wall of the respi- 

 ratory chamber breaks up into a lattice- 

 work, the fine slits of which, arranged in 

 rows, are beset with cilia. In the rods of 

 the lattice-work blood-channels are exca- 

 vated. Water passes through the slits into 

 the peribranchial spaces formed by the out- 

 growth of the above-mentioned sacs, whence 

 it is conducted to the cloaca, and thence to 

 the common excurrent aperture. 



In the compound Ascidians the excur- 

 rent apertures of a number of individuals 

 are united to form a common cavity, so that 

 each group of individuals possesses a single 

 excurrent aperture placed in the centre and 

 surrounded by a number of incurrent aper- 

 tures. The entrance into the respiratory 

 chamber is, particularly in the Ascidians, surrounded by tentacular 

 organs, which are partly in the form of external processes, partly 

 placed at a distance from the orifice, and are pointed towards it. 

 The lattice-work of the gills affords an endless variety in the 

 arrangement of its component rods, and in the form and number of 



~ e 



Fig. 210. Diagram of an 

 Ascidian. o Incnrrcnt ori- 

 fice, i Respiratory chamber, 

 c Ventral groove, n Gan- 

 glion, d Digestive canal. 

 cl Cloaca. g Generative 

 gland. 



