ENTEROPNEUSTA 385 



seems to indicate the paired origin of even this anterior 

 coelomic sac. The acorn pore arises from the dorsal pore 

 of the Tornaria. In the Balanoglossus which does not pass 

 through the Tornaria stage, the anterior coelomic sac, as 

 Bateson unreservedly calls it, forms a pointed process, the 

 end of which fuses with the ectoderm and breaks through 

 to the exterior. 



After the coelomic sacs have separated from the archen- 

 teron, the remaining entoderm produces, in the form of a 

 forward evagination, the intestinal diverticulum, which lies 

 at the base of the acorn (comp. Fig. 165 di), and from 

 this the formation of the acorn stalk probably takes place. 

 Even earlier than this, the gill-pockets develop as paired 

 evaginations from a portion of the intestine behind the 

 diverticulum. They are dii'ected toward the dorsal surface 

 (Fig. 169 B), with which they soon unite, since they open to 

 the exterior by means of pores which are at first rather large 

 (Fig. 168 C). In several forms at first only one pair of gill- 

 pockets is to be observed (Figs. 168 G and 169 B) ; in the 

 Tornaria studied by Agassiz, on the contrary, four pairs of 

 them make their appearance simultaneously (Fig. 171). 

 The form of the gill-pockets, which is at first so simple, 

 is later much more complicated, for their walls become 

 folded, and the skeletal hoops are developed between them. 

 The formation of new gill-pockets continues to take place 

 even when the Balanoglossus has long since assumed its 

 permanent shape. After the posterior part of the body has 

 considerably increased in length, the paired evaginations of 

 the intestine which have been interpreted as hepatic append- 

 ages ai'ise behind the gill region. 



In the Tornaria there is found next to the so-called water-vascular 

 vesicle, or even sunk into a depression in it, a spherical vesicle, which 

 ordinarily is called the heart of the Tornaria (Figs. 169 B and 171 

 ht [not h, Fig. 1G5] ). It does not merit this name, for, according to 

 Spengel, it is developed into the organ which Bateson and Kohler have 

 called the saccular posterior portion of the "proboscis gland" (comp. 

 Fig. 165). In Balnnoijlossus Kowalevskii the proboscis gland arises by 

 delamination from the tissues of the anterior coelomic sac after this has 

 already spread out inside the acorn. This mode of origin seems to us 

 K. H. E. CO 



