STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 115 



different sections, which have been treated in different ways. The two figures 

 are drawn to scale, and therefore represent the true relative sizes of the bars. 



Fig. 1. A transverse section of a tongue bar of the pharynx of Amphi- 

 oxus (Branchiostoma lubricum, Costa). It represents, accu- 

 rately as I believe, a typical section of the bar. Most of the parts 

 are fully named on the plate. The rod contains a cavity — the coelom, 

 lined by an epithelium, whose nuclei are shown (see also Fig. 13). 

 Within this ccelom lies a blood-vessel — the skeletal vessel. Two other 

 blood-vessels are present in the bar, the "visceral" (Fisc. Bl. vessel) 

 and the "somatic" blood-vessel; these lie at either end of the septal 

 membrane, between the two layers of which are shown three nuclei 

 (see also Figs. 20, 21). The atrial epithelium consists of only one 

 kind of cell. The grouping of the epithelial nuclei at the pharyngeal 

 end of the bar may be noted. 



Fig. 2. A transverse section of a primary bar (see explanation of Fig. 1). 



Here the coelom is more extensive, and the "cutis" — or outer wall of 



the coelom — is thinner than in the tongue bar. The rod itself is made 



up of two pieces, meeting along the middle line of the bar, and a 



third piece wedged between. The skeletal blood-vessel is outside the 



coelom, but is in reality surrounded by the rod, i.e. "cutis." The 



atrial epithelium consists of two kinds of cells (see Fig. 17), viz. 



a, the vacuolated, and b, the granular cells. 



Figs. 3 to 12 are tracings of figures published by previous observers; the 



only alterations that have been made are (1) colouring the rod yellow, and (2) 



filling in with black the spaces regarded as blood-vessels by the authors. 



Fig. 3. Copied from Stieda's fig. 6, pi. i. " Transverse section of a 

 branchial plate, a. The rod. b. Deep layer of the epithelium, c. 

 Superficial layer with cilia." 



Fig. 4. Copied from Langerhans, fig. 24, pi. xiii. " Transverse section 

 througli a gill bar. m. Epithelium, k. Nuclei of branchial epithelium 

 /j.e. Pigmented epithelium, h. The atrial epithelium. 5. Hollow gill rod." 



Fig. 5. Copied from Schneider's fig. 4, pi. xiv. "Transverse section 

 of a thin (that is, tongue) gill bar. a. Triangular space, a blood- 

 vessel. A. A blood-vessel in communication with the branchial artery. 

 k^. The rod. p.p. Peritoneal (i. e. atrial) plate." 



Fig. 6. Copied from Lankester, pi. xxxvi b, fig. 2. " Transverse 

 section of a tongue bar. al. Left inner epithelial band. ar. Right 

 inner epithelial band. am. Median inner epithelial band. col. Columnar 

 lateral cells, with long cilia, n. Superficial nuclei. «i. Deeper nuclei. 

 sept. Clear septal tissue. Bl. v. Supposed blood-vessel, ending blindly 

 at the ventral extremity, pig. Lateral groups of pigment in the atrial 

 epithelium, at. ep. Atrial epithelium (epidermic)." 



Fig. 7. Copied from Spengel, fig. 19, pi. xviii. " Transverse section of 

 a tongue bar. vz. Chief vessel of the bar. vn. Accessory vessel." 



