876 EMBRYOLOGY 



The appendicular structures which arise from the intestine, 

 and in part remain intimately united with it, are important 

 for the organization of the animal. The intestine produces 

 in its anterior part a dorsal evagination, which extends into 

 the base of the acorn (Fig. 165 di). Between the ventral 

 wall of this evagination and the epidermis of the acorn is 

 inserted the anterior part of the so-called proboscis- or acorn- 

 framework (Fig. 165 si-), a skeletal body, the unpaired part 

 of which has the position described, whereas two arms, 

 which project from it backwards and downwards, embrace 

 the fore-gut like a hoop. They lie in folds of the intestinal 

 wall ; they could not be introduced into the figure. The 

 entire skeletal bod^^ according to Kohler, Bateson, and 

 Morgan, is a product of the intestinal epithelium, i.e., the 

 above-mentioned evagination of it, [whereas Spengel (No. 

 VI.) interprets the acorn-skeleton as only a modification of 

 the bounding membrane (^Grenzviembran), and also makes 

 the coelomic sacs share in its formation. According to 

 Spengel, the histological structure of the acorn-skeleton in no 

 way agrees with that of the chorda doi^salis of vertebrates ; 

 notwithstanding, this has been repeatedly emphasized by 

 different observers, and has been regarded as one of the 

 points for comparison between Enteropneusta and Vertebrata 

 (comp. infra). — K.] 



The gills, which are most important for the whole inter- 

 pretation of the animal, occur somewhat further back on the 

 intestine, behind the collar region of the body. They 

 are paired, pouch-like outfoldings of the dorsal wall of 

 the intestine lying on both sides of the middle line (Fig. 166 

 k). Each of these pockets, which are lined with cilia, sends 

 upwards a short duct, which opens on the dorsal surface by 

 means of a pore (Fig. 166 /)). Externally the rows of 

 branchial pockets can be recognized by the transverse arched 

 bands (Fig. 164 A). Behind, these transverse arches become 

 less extensive, which indicates that the formation of new 

 gills takes place even in the later stages of the animal's life. 

 A skeleton, formed of delicate chitinous hoops, which is 

 embedded in the walls of the branchial pouches, serves as a 

 protection for the gills. The water is taken in by the mouth, 



