662 DESCRIPTION OF PLATES, 24, 25. 



Fig. 5, Diagram, illustrating the supposed changes in position of the 

 gnathites in Proteolepas, causing them to stand back to 

 back ; a, outer maxilla ; b, maxilla ; c, mandibles ; d, 

 lab rum. 



PLATE 25. 



Proteolepas bivincta. 



Fig. 6, Proteolepas bivincta : diagram, showing the probable position of 

 the young animal, just before its metamorphosis, within 

 the carapace of the supposed pupa ; a, caudal appendages ; 

 b, six pairs of natatory thoracic legs ; c, mouth, no doubt 

 closed and functionless, as in other pupae ; g, threads of 

 attachment, with cement-ducts in process of formation ; 

 h y antennae, 

 7, Proteolepas bivincta, magnified about twenty-six times. 



m, mouth, the summit being formed of the lab rum and palpi 

 joined. 



V, first segment of body ; homologically the seventh or last 

 cephalic segment. 



2 t to 8 t, second to eighth segment of body ; homologically, first 

 to seventh thoracic segments. 



9 « to u a, ninth to eleventh segment of body ; homologically, 

 three segments of abdomen. 



d', three muscles attached on each side to the labrum, and 

 running to the gnathites. 



e e, great ovarian sack. 



f t true ovaria. 



g, threads of attachment. 



h t three terminal segments of the antennae of the pupa. 



iy vesicula seminalis. 



k, papilla representing the penis. 



Fig. 1, Balanus tintinnabulum : an enlarged longitudinal section 

 through the shell and sack, with the right-hand scutum and 

 tergum and right-hand half of shell and basis removed, ex- 

 hibiting the body of the animal not in section. The cirri 

 are exhibited only on one side. 



a, a, orifice of shell, within which lies the operculum formed 

 by a pair of scuta (s), and pair of terga (t). 



b, basis (homologically the anterior end of the shell). 



k, carina of shell (or dorsal valve or compartment of shell). 



k', sheath of carina. 



L, lateral compartment of shell. The carino-lateral compart- 

 ment is hidden by the scutum and tergum. 



r, rostrum of shell (or ventral valve or compartment of shell). 



r', sheath of rostrum. 



o, o, opercular membrane, connecting the opercular valves with 

 the overhanging basal edge of the sheath. 



s, scutum. 



