68 ON A TOENAEIA FOUND IN BRITISH SEAS. 



DESCKIPTION OF PLATES YII, YIII, 



Illustrating Mr. Gr. C. Bourne's paper on ^' A Tornaria found in 



British. Seas." 



Fig. 1. — Lateral view of a very young Tornaria (?). 



Fig. 2. — Longitudinal section through the same specimen, showing the relation of the 

 stomodseum to the gut. 



Fig. 3. — Posterior part of the next section to 2, showing the blastopore persisting as the 

 anus. 



Fig. 4. — Young Tornaria, older than fig. 1. Actual size "33 mm. 



Fig. 5. — Frontal section through the posterior region of a Tornaria of the same age as 

 fig. 4, to show the communication between the mid-gut and the hind- gut. 



Figs. 6, 7, 8, 9, 10. — Five consecutive sections through a perfect Tornaria in tlie region 

 of the proboscis pore, showing the opening of the latter {d. p.) and the canal (ant. b. c.) 

 which connects it with the proboscis cavity. In figs. 6, 7, 8, the mode of origin of the so- 

 called heart (v.) may be seen. 



Fig. 11. — Surface-view of the circumanal ciliated ring, showing the nuclei of the ciliated 

 cells, the supporting cells, and the pigment spots. 



Fig. 12. — Sagittal section through a Tornaria of the same age as fig. 4, showing the 

 proboscis cavity and pore. 



Fig. 13. — Lateral view of a perfect Tornaria, showing the complication of the ciliated 

 bands. Actual size 1 mm. 



Fig. 14. — Longitudinal section through the posterior region of a Torn../ia of the same 

 age as fig. 3, showing the origin of the mesoblast from the hind-gut. 



Fig. 15. — Longitudinal section through a somewhat more advanced larva, showing the 

 mesoblast separate fi'om the hind-gut, and closely applied to the wall of the mid-gut. 



Fig. 16. — Transverse section of a larva somewhat older than fig. 15. A cavity has been 

 formed in the mesoblast. 



Fig. 17. — Surface view of the ectoderm in a perfect Tornaria. 



Fig. 18. — Frontal section through the apical sense organ, showing the sense pits (s.p.), 

 the ganglion-cells and layer of nerve-fibres, the proboscis cavity, and the muscle band con- 

 necting the latter with the sense organ. 



Fig. 19. — Section through the proboscis gland of an advanced larva of a Balanoglossus 

 from the Bahamas, showing the relations of the so-called heart (proboscis sac) to the 

 proboscis gland. Copied from a drawing by Mr. Weldon. 



Lettering in the above figures. 



a. Lateral loop of the pre-oral band. a'. Median loop of the pre-oral band. a". Lateral 

 loop of the longitudinal band. a'". Median loop of the longitudinal band. a. b. Circum- 

 anal ciliated band. am. c. Amoeboid cells, an. Anus. ant. b. c. Proboscis cavity, d. p- 

 Proboscis pore. ect. Ectoderm. end. Endoderm. g.c. Ganglion-cells, h.g. Hind-gut. 

 I. b. Longitudinal band. m. Mouth, m. b. Muscular band. mes. Mesoblast. m. g. Mid- 

 gut, n.fi. Layer of nerve-fibres, ces. Oesophagus, pr. b. Pre-oral band. proe. Procto- 

 dseum. p.s. Pigment spots, s.c. Supporting cells, s. o. Apical sense organ, s.^. Sense 

 pits. V. " Heart," or proboscis sac. 



