PLATE 25.
Genera—PROTEOLEPAS AND BALANUS.

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 pupæ; g, threads of attachment, with cement-ducts in process of formation; h, antennæ.
• 7, Proteolepas bivincta, magnified about twenty-six times.
• m, mouth, the summit being formed of the labrum and palpi joined.
• ¹c, first segment of body; homologically the seventh or last cephalic segment.
• ²t to ⁸t, second to eighth segment of body; homologically, first to seventh thoracic segments.
• ⁹a to ¹¹a, 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, true ovaria.
• g, threads of attachment.
• h, three terminal segments of the antennæ of the pupa.
• i, 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, exhibiting 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 compartment 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.

• T, tergum.
• a, adductor scutorum muscle, with the scutum on the near side removed.
• b, the whole space enclosed by a broken sinuous line, round a and b, shows the cut surface of attachment to the near scutum, which has been removed.
• b′, lower muscle, on the near side, running from near the basal edge of the scutum to near the basal margin of the labrum (e). Above this are three other muscles (all on the near side), running to the membrane between the labrum and adductor muscle.
• c, prosoma, including the main portion of the stomach and thickened ends of the vesiculæ seminales: homologically this is formed by the development of the second thoracic segment, which carries the first pair of cirri; and possibly, in part, by the antecedent segment, i. e. the first thoracic segment.
• c′, thorax, extending from the prosoma to the posterior end of the body: the letter (c′) stands on the segment supporting the third cirrus; homologically, this segment is the fourth thoracic.
• d′, orifice of the acoustic sack, above which is the basal articulation of the first cirrus.
• e, labrum, forming the back (i. e. anterior end) of the mouth, with the transverse palpi obscurely seen on the summit.
• f, sack in which the animal lies, with the tunic lining it, continuous with that investing the prosoma (c), and lining the under sides of both opercular valves, but cut off round (b) and (a) by the removal of the near scutum. The branchia on the further side, which occupies the position represented at fig. 3, is covered by the body of the animal.
• g g, ovarian, inosculating cæca, branching from the simple duct (of which the near one of the pair is represented), leading to the ovaria (not represented) within the body.
• h, rostral depressor muscle of the scutum: the lateral depressor muscle of the scutum is hidden by the body and by the membranes of the sack.
• i, carinal depressor muscle of the tergum.
• z, antennæ (three terminal segments) of the pupa; I distinctly saw these in this species, but they are here represented considerably too large, even on the supposition that a young shell had been drawn, and magnified about twelve times.
• 2, Testes of Balanus perforatus, greatly magnified.
• 3, Branchia of Bal. tintinnabulum, on the same scale as in fig. 1, and in its natural position. This drawing was made by lifting up the body in fig. 1; the organ being thus completely exposed over its whole interior surface; a is the basal end of the spur of the tergum.