56 CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. [April 4, 



Glottidia, but this system is already well formed in the earliest 

 larva which he observed. 



7. Enteron. — The cavity of the enteron is flask-shaped, the 

 enlarged end lying in the cephalic region while the pointed ex- 

 tremity extends into the peduncle. In early larval stages the 

 transverse diameter of the enteron is greater than its dorso-ventral 

 diameter (Figs. 25 and 44-50) ; in later stages it becomes circular 

 in cross section (Figs. 53-55), while in still later stages it becomes 

 compressed laterally so th'at its greater diameter is in a dorso-ven- 

 tral direction (Figs. 33, 35, 56-60). In the oldest stages which I 

 have observed a diverticulum from the anterior end of the enteron 

 grows out toward the ventral side, and at the same time an invagi- 

 nation of the ectoderm appears in the anterior mantle furrow, at the 

 very point where the blastopore remnant disappeared, and grows in 

 toward this diverticulum (Figs. 35 and 58). I presume that this is 

 the first step in the formation of the oesophagus. During the whole 

 of the larval period the enteron has no opening to the exterior. 

 According to Morse ('73; the mouth is formed late in the meta- 

 morphosis, and while he does not describe the place or manner of 

 its formation his Fig. 90 shows it at the anterior extremity of the 

 young brachiopod. Kowalevsky (' 74) also has described and figured 

 what he considers to be the formation of the oesophagus at the 

 anterior end of the cephalic lobe. From what I have observed I 

 feel confident that the mouth is first formed on the ventral side, in 

 the region of the anterior mantle furrow, and if it is later found at 

 the anterior end of the young brachiopod, its change in position 

 must have been brought about by secondary changes. Kowalevsky 

 expresses some doubts as to whether the invagination observed by 

 him at the anterior end of the cephalic lobe is really the oesopha- 

 gus, and from its location I would suggest that it is the apical sense 

 plate and cerebral ganglion rather than the oesophagus. My obser- 

 vation as to the location of the stomodaeal invagination thus con- 

 firms Heider's ('93) theoretical suggestion and brings the brachiopod 

 larva into close relationship with the trochophore. 



The cells bounding the lumen of the enteron are cuboid in early 

 stages and columnar in later ones (compare Figs. 43-50 with Figs. 

 56-60). These cells, enclosing a minute lumen, extend through 

 the entire peduncular region (Figs. 2i^, 57, 58, 61-64). 



From its earliest formation the enteron is in contact on its dorsal 



